Session 5: Chemical Evolution

00:00:00.00 My name is Jack Szostak,
00:00:01.25 I'm a Professor of Genetics at Harvard Medical School,
00:00:05.22 I'm an Investigator at Massachusetts General Hospital, where my labs are,
00:00:09.18 and I'm also an Investigator of the Howard Hughes Medical Institute.
00:00:14.17 In this lecture, what I'd like to tell you about is recent advances
00:00:21.06 in work from my lab on the origin of cellular life on the early Earth.
00:00:26.22 But before I get into those experiments,
00:00:29.08 I'd like to step back from the origin of life per se,
00:00:33.08 and talk a little bit about some insights from modern biology
00:00:39.00 that bear on this question,
00:00:41.01 in particular why the question has attracted so much interest and attention recently.
00:00:46.16 So, this is one of the iconic images of hydrothermal deep sea vents.
00:00:56.02 This is an environment characterized by very high temperature and pressure,
00:01:01.21 and of course the surrounding area is just teeming with life.
00:01:07.04 Here's another example: an image from Norm Pace.
00:01:12.04 You can see a layer of green cells growing inside the rock.
00:01:18.07 These are photosynthetic cyanobacteria,
00:01:21.29 and they're living in the pores of the rock at very low pH.
00:01:28.23 This is one of the famous hot springs in Yellowstone National Park.
00:01:34.26 Again, a very high-temperature environment; again, full of life.
00:01:43.29 And here's yet another distinct kind of extreme environment,
00:01:48.21 another very low pH environment.
00:01:50.14 This is the Rio Tinto in Spain.
00:01:53.27 Very acidic water, but again teeming with life:
00:01:59.00 microbial, eukaryotic life.
00:02:04.17 There are even more extreme examples of this kind of environment in acid mine drainage sites,
00:02:12.00 where the water that's flowing out is basically sulfuric acid at a pH close to zero.
00:02:18.24 And again there is microbial life.
00:02:21.11 So with all of these examples,
00:02:22.22 what it's telling us is just the remarkable extent which our planet
00:02:27.21 has been colonized by life.
00:02:31.17 And even environments that we would've considered incredibly hostile and extreme
00:02:38.20 are apparently easily adapted to by life.
00:02:43.10 And of course, this is a consequence of the power of Darwinian evolution,
00:02:48.10 to lead to adaptations to diverse environments.
00:02:53.16 So, if you put this together with recent observations
00:03:00.01 from our astronomy colleagues, in terms of the discovery of extrasolar planets,
00:03:06.02 it really puts into focus the question of whether there is life out there,
00:03:13.13 apart from our planet.
00:03:16.07 So this is an image of the Milky Way, of course.
00:03:22.20 Up to a couple of years ago,
00:03:24.21 astronomers had discovered on the order of 500 extrasolar planets,
00:03:29.04 planets orbiting other stars.
00:03:32.08 But more recently, as a result of the Kepler mission,
00:03:36.00 a space telescope that is just pointed continuously at a very dense starfield,
00:03:45.22 a large number of additional planets have been found,
00:03:48.24 about 1200 candidates at the last count.
00:03:52.28 And these are detected as the planets orbit around their star,
00:03:57.29 and if they eclipse the star, if they transit in front of it,
00:04:00.27 they block out some of the light, and you can detect that little dip in the intensity of the light.
00:04:06.23 So this has given us a big enough sample to actually make extrapolations,
00:04:12.02 and what I've heard from scientists associated with the Kepler mission
00:04:17.17 is that those extrapolations suggest that there could be roughly on the order of
00:04:23.06 500 million, perhaps even a billion, Earth-like planets
00:04:27.00 orbiting sun-like stars out there in our galaxy.
00:04:32.13 And so, if you put that together with the fact that we know,
00:04:37.03 on our planet, that at least microbial life can live in incredibly harsh and diverse environments,
00:04:44.28 it's pretty clear that there will environments out there on
00:04:49.12 these other planets that could support life.
00:04:52.03 So the question is, and the thing we all really want to know is:
00:04:56.01 Is there life out there?
00:04:58.01 Are we alone, or is the universe, is our galaxy, full of life?
00:05:03.15 So this really comes down to the question you see here.
00:05:07.25 Is it easy or hard for life to emerge from the chemistry of early planets?
00:05:14.13 And, unfortunately, it's going to be a long time before we can answer that question
00:05:18.28 in the most satisfying way, by direct observation.
00:05:23.16 Even to get indirect evidence from spectroscopy of planetary atmospheres
00:05:30.27 may take 10, 20, 50 years, to look at Earth-like planets.
00:05:38.28 So what can we do in the meantime to try to get some clues to answer this question?
00:05:46.27 So, what we've been doing, and other people have been doing,
00:05:49.29 is to go into the lab and do simple, chemical experiments
00:05:54.29 and try to work out a complete, step-by-step, plausible pathway,
00:06:02.17 all the way from simple chemistry to more complex chemistry to simple biology.
00:06:08.02 And if we can actually show that there's a continuous pathway
00:06:13.26 with no super-hard steps along the way,
00:06:17.20 then I think we can conclude that it's likely that there is abundant life
00:06:22.14 out there in our galaxy.
00:06:25.03 On the other hand, it could be that our experiments show
00:06:29.14 that there are some steps in that pathway that are extremely difficult,
00:06:35.01 there are bottlenecks that might be very hard to overcome,
00:06:38.29 in which case the emergence of life might actually be a very rare phenomenon.
00:06:44.04 And in the extreme, we could be it.
00:06:47.23 This could be the only place in our galaxy or even the universe where life has emerged.
00:06:53.28 So we would like to try to get some insight into these questions
00:06:58.11 by doing simple laboratory experiments.
00:07:00.20 Now, there's a related question, which is shown down here.
00:07:06.03 If there is life out there,
00:07:07.25 is it likely to be pretty similar to what we're familiar with on our planet?
00:07:16.12 Will life that evolved independently elsewhere have the same
00:07:23.25 fundamental kind of biochemistry?
00:07:25.24 Will it be cells that are living in water,
00:07:29.00 using if not RNA and DNA, some nucleic acid to mediate heredity?
00:07:35.20 Will they use protein-like molecules to carry out biochemical functions?
00:07:41.18 Or could there be forms of life that are actually much different, much more diverse,
00:07:49.14 maybe using completely different kinds of molecules to mediate heredity
00:07:52.25 and to mediate function?
00:07:55.04 Or even forms of life that live in very different environments,
00:08:00.06 for example, in solvents other than water.
00:08:03.23 Again, this is the kind of thing that we can address by going into the lab
00:08:09.17 and doing simple experiments, and trying to build structures,
00:08:12.26 and assess the possibility of having living systems in
00:08:18.09 different kinds of environments and with different molecular bases.
00:08:25.16 So, let's try to think, then,
00:08:30.21 about how we can deduce something about early forms of life.
00:08:37.04 After all, if we want to experimentally investigate the beginnings of life,
00:08:41.26 we have to have some idea, some kind of conceptual model,
00:08:45.22 of what very primitive forms of life looked like.
00:08:50.17 And this has been a very difficult thing for people to think about,
00:08:55.03 because we're so biased by our view and our understanding of modern life.
00:09:00.25 So if we look at modern cells, they're incredibly complicated:
00:09:05.16 Just a lot of moving parts, very elaborate structures,
00:09:09.14 such as you can see here in this elaborate structure in a eukaryotic cell,
00:09:15.15 all the machinery involved in cell division.
00:09:19.00 If you go deeper and look at the underlying biochemistry,
00:09:23.09 if anything, it's even more complicated.
00:09:26.17 And this is just a small section of the chart of central metabolism,
00:09:32.09 so there are hundreds or thousands of enzymes that catalyze
00:09:36.22 all of the metabolic reactions that are required for cells to grow and divide.
00:09:43.10 Even the general organizational structure of modern cells is very complicated,
00:09:51.10 in the sense that it's highly self-referential.
00:09:54.24 So every aspect of this process,
00:09:59.03 this central dogma (the transmission of information from DNA to RNA
00:10:03.10 to proteins and then down to building structures with function),
00:10:08.21 every part of that depends on all the other parts.
00:10:12.12 So for example, the replication of DNA requires DNA,
00:10:16.14 but it also requires RNA and proteins, the polymerases.
00:10:20.11 The transcription of RNA requires DNA,
00:10:25.11 which is where the information's stored, but it also requires many proteins,
00:10:29.21 and it also requires many other RNA molecules.
00:10:33.02 And similarly, the formation of proteins occurs on a remarkably complex machine,
00:10:38.24 the ribosome, which is itself composed of RNA and proteins.
00:10:44.02 So, for decades, it was very hard for people to think of any reasonable way
00:10:51.07 in which such an internally self-referential system could emerge
00:10:57.25 spontaneously from a chemical environment.
00:11:01.16 And the answer to that really came from thinking about RNA
00:11:08.27 and the different things that it can do.
00:11:10.21 So this simplification in thinking came from the realization that RNA can not just
00:11:18.04 carry information but can also catalyze chemical reactions.
00:11:23.15 And that realization led immediately to the hypothesis that,
00:11:29.01 in primitive cells, RNA might be able to catalyze its own replication,
00:11:34.05 also carry out biochemical functions for the primitive cell.
00:11:39.10 And so then all you really need to think about is a cell with RNA molecules
00:11:45.14 encapsulated within some kind of primitive cell membrane
00:11:48.25 that itself could be a self-replicating structure.
00:11:52.08 So, the history of this idea actually goes back to the 1960s,
00:11:58.24 and three very smart people, Leslie Orgel, Carl Woese, Francis Crick,
00:12:05.05 hypothesized in part on the basis on the complex folded structure of tRNA,
00:12:13.01 that an early stage of life might've evolved RNA as the
00:12:19.01 sole macromolecular basis of evolved machinery.
00:12:24.25 And so, this lets you think of simple cells emerging with just a single biopolymer,
00:12:31.19 RNA, and that later on, as evolution
00:12:38.03 developed more complex cellular structures, information storage
00:12:42.14 became specialized in DNA,
00:12:44.18 and most functional activities because specialized as the job of proteins.
00:12:51.06 Now, although these ideas were put forth in rather elementary form in the 60s,
00:12:56.14 of course nobody took them seriously at the time,
00:12:59.21 because there was absolutely no experimental evidence for the idea
00:13:04.08 that RNA could catalyze chemical reactions.
00:13:06.23 At the time, people had just started to get very detailed,
00:13:11.19 high-resolution information about how proteins catalyzed reactions,
00:13:16.13 and the idea that a molecule like RNA could do the same thing seemed ludicrous.
00:13:22.26 So it wasn't until almost 20 years later,
00:13:25.05 with the work of Tom Cech and Sid Altman,
00:13:29.10 and the experimental demonstration that RNA molecules could actually
00:13:34.02 very effectively catalyze at least certain types of chemical reactions,
00:13:38.14 that people took this whole idea of an RNA-based early stage of life seriously.
00:13:45.00 And so that hypothesis, the "RNA world hypothesis,"
00:13:48.23 was really summarized by Walter Gilbert in an article in 1986,
00:13:57.18 and this has really become the foundation of a lot of thinking
00:14:02.12 about early stages in the emergence of life.
00:14:08.01 So, apart from the basic facts,
00:14:11.20 that RNA does and can catalyze chemical reactions,
00:14:15.04 is there any other evidence that early life might have been
00:14:19.17 based more exclusively on nucleic acids?
00:14:24.21 And in fact, there are several lines of circumstantial evidence.
00:14:28.24 So one of them is the structure of many cofactors.
00:14:33.07 So here you see acetyl-CoA, just one example.
00:14:39.03 But the working part of the molecule is the thioester out here,
00:14:43.27 and for no obvious reason, there's a nucleotide at the other end.
00:14:48.13 And really the only way to make sense of that is the nucleotide is a "handle,"
00:14:53.11 either a relic of a primitive ribozyme
00:14:56.24 or something that was easy for primitive ribozymes to grab hold of and thereby,
00:15:04.10 using this cofactor, catalyze reactions in a thioester-mediated way.
00:15:11.14 Now there are other examples.
00:15:14.21 Here is vitamin B12, another very important catalyst.
00:15:20.24 Its working part is this complex corrin ring,
00:15:25.07 but down here you see, again, a nucleotide.
00:15:29.22 What's it doing there?
00:15:30.28 It's probably another relic of the RNA world,
00:15:34.02 when all of this complicated biochemistry was being catalyzed by RNA enzymes.
00:15:40.12 Yet another example is the very way that the substrates for
00:15:45.25 DNA synthesis are made, and they're not made de novo,
00:15:50.29 as you might expect if DNA came first.
00:15:53.27 They're actually made from preexisting ribonucleotides,
00:15:58.02 and so the transformation of ribonucleotides to deoxynucleotides
00:16:03.17 is catalyzed by the enzyme ribonucleotide reductase.
00:16:08.13 And this unusual synthetic pathway can be viewed as the relic of the fact that,
00:16:17.04 early in time, metabolism and RNA synthesis used ribonucleotides,
00:16:23.25 and only later when DNA was invented or evolved,
00:16:29.10 was there was requirement to make deoxynucleotides,
00:16:32.15 and so they're from the closest available substrate.
00:16:38.03 Finally, perhaps the most important and dramatic piece of evidence
00:16:44.14 for the early role of RNA in primitive forms of life
00:16:50.06 is the actual structure of the ribosome.
00:16:53.21 And so this is a slide from Tom Steitz showing a
00:16:57.27 view into the active site of the large subunit.
00:17:02.23 So this is the peptidyl transferase center,
00:17:05.06 and this little green structure in here is a transition state analogue
00:17:09.17 that marks it at the place in this giant machine where the chemistry is happening.
00:17:14.28 And what you can see is that it's these gray squiggles,
00:17:17.29 which are the RNA, that completely make up that active site.
00:17:23.23 So all proteins are generated by an RNA machine,
00:17:29.19 the RNA central region of the ribosome itself.
00:17:34.25 So again, this only makes sense in terms of an early stage of biochemistry
00:17:40.00 dominated by RNA functions, which then over time evolved the ability
00:17:45.15 to make proteins, which are now so important in all modern biochemistry.
00:17:52.05 So, if we want to understand the origin of life,
00:17:57.03 what we need to think about is not simply how to make
00:18:02.14 these incredibly complex modern cells, but we need to think about how to go from chemistry
00:18:07.23 to very simple, RNA-based cellular structures.
00:18:13.08 So, what would the process look like?
00:18:16.04 What's the broader picture?
00:18:17.17 When did this all happen on the early Earth?
00:18:20.17 So, what was the timeframe in which these events took place?
00:18:25.07 This is a slide from a review by Gerald Joyce, and it summarizes
00:18:30.02 the broad sweep of events that were important in the origin of life.
00:18:34.20 So we actually need to think of everything from planet formation,
00:18:38.14 the beginning of the Earth itself around 4.5 billion years ago;
00:18:44.07 over time as the Earth cooled, water could condense,
00:18:48.01 we have a stable hydrosphere, we have liquid water on the surface;
00:18:52.06 following that, increasingly complicated organic chemistry going on,
00:18:57.08 probably in many different environments on the early planet;
00:19:01.00 and then somehow that led up to the synthesis of RNA
00:19:06.09 or RNA-like molecules on the Earth,
00:19:09.13 which could start to carry out biochemical functions inside primitive cells;
00:19:14.24 and then eventually lead to the emergence of much more complicated cells
00:19:19.29 that would be biochemically similar to modern life.
00:19:23.14 Now, the first really good evidence we have about
00:19:27.04 the appearance of modern microbial life is roughly 3.5 billion years ago,
00:19:33.17 so there's a billion-year interval between the formation
00:19:38.17 and cooling of the planet and the first good evidence for life.
00:19:43.21 And basically, we have very little hard evidence about
00:19:48.00 where all of these important events that led up to life emerging from chemistry,
00:19:53.01 when they actually happened.
00:19:55.03 And that goes along with the fact that we have very little concrete evidence
00:19:59.25 concerning the environments in which those transitions took place.
00:20:04.27 So, this is one of the difficult aspects of studying this question.
00:20:09.12 We can't actually go back,
00:20:10.28 we can't know for sure what the early environments were really like,
00:20:16.13 we'll never know exactly what really happened.
00:20:19.15 So what's our goal in studying these questions?
00:20:23.00 What we're trying to do is really come up with a plausibly realistic
00:20:29.16 sequence of events so that we understand all of the transitions
00:20:35.00 throughout this whole pathway,
00:20:37.05 and we'd like to understand a complete pathway,
00:20:38.29 from planet formation through early chemistry,
00:20:41.28 more complicated organic chemistry,
00:20:44.05 up to the assembly of those building blocks into the first cells,
00:20:48.29 the emergence of Darwinian evolution,
00:20:51.04 and then the gradual complexification of early life leading up to what we see now.
00:20:58.10 So, let's look a little bit more closely at the chemical steps.
00:21:03.23 So in broad outline, what we think happened is that you
00:21:07.17 start off with very simple molecules such as shown up here.
00:21:14.02 There's still a lot of debate about the nature of the early atmosphere.
00:21:20.00 Scientific opinions have gone back and forth in terms of
00:21:23.09 the structure and how reducing that atmosphere was.
00:21:27.26 But it's also been recognized that there could be very important local variation,
00:21:32.22 so even if the atmosphere was globally fairly neutral
00:21:38.20 or perhaps mildly reducing or mildly oxidizing,
00:21:41.16 there could be local environments that were more reducing.
00:21:46.14 That, together with the input of various forms of energy
00:21:51.21 (for example, from electric discharges, lightning,
00:21:56.06 high-energy ultraviolet radiation, ionizing radiation)
00:22:01.08 these are all forms of very energetic processes that can basically
00:22:06.15 rip these small starting molecules apart into atoms,
00:22:11.26 which can then recombine to generate high-energy intermediates
00:22:15.17 with multiple bonds, molecules like cyanide and acetylene,
00:22:20.19 formaldehyde and so on.
00:22:22.18 And these molecules can then start to interact with each other
00:22:26.20 and gradually build up more complex intermediates,
00:22:29.13 ultimately leading to the things we really care about:
00:22:33.10 the lipids that will make membranes and vesicles,
00:22:37.25 the nucleotides that will assemble into genetic molecules like RNA,
00:22:42.16 amino acids that can assemble into peptides,
00:22:45.29 which may also play roles in primitive cells.
00:22:49.10 And somehow, and this is the question that my lab has really been focused on,
00:22:53.12 somehow all of these molecules come together
00:22:56.17 and assemble into larger structures that look and act like cells
00:23:01.10 that can grow and divide.
00:23:04.04 So how could that possibly happen,
00:23:06.17 and what would such a primitive cell look like?
00:23:09.24 So here is a schematic version of the way that we're thinking
00:23:13.21 about a primitive cell, or "protocell."
00:23:17.11 So what we think are the important components of a primitive cell
00:23:22.14 are basically two things:
00:23:25.01 a cell membrane and inside,
00:23:28.10 some kind of genetic material, maybe RNA, maybe DNA,
00:23:32.05 maybe something simpler, something more stable, we're not really sure.
00:23:37.12 So the first question is how could you assemble such composite structures?
00:23:43.23 So we want to be have a membrane boundary
00:23:46.13 that can keep important molecules encapsulated within
00:23:51.06 and essentially provide a distinction between the cell itself
00:23:54.22 and the rest of the universe.
00:23:57.05 We need to understand how these two components self-assemble,
00:24:02.09 how they come together.
00:24:04.07 And it actually turns out that that part is all fairly straightforward.
00:24:09.26 Self-assembly processes are critical in thinking about all of the steps,
00:24:15.12 and there are multiple different ways in which these components
00:24:19.06 can be made and can come together.
00:24:22.08 A much harder question and more interesting is:
00:24:28.08 Once you have structures like this,
00:24:30.16 how can they grow and then divide without any of the
00:24:34.17 complicated biochemical machinery that's present in all of modern life?
00:24:40.25 So since we're talking about the origin of life,
00:24:43.00 then by definition we didn't have highly evolved biochemical machinery around.
00:24:48.21 So it's sometimes hard to think about these problems
00:24:51.17 because modern cells use so much biochemical machinery
00:24:56.11 to mediate the process of cell growth and cell division.
00:25:00.12 It's almost hard to think of how could that be driven
00:25:06.04 by simple chemical and physical processes.
00:25:10.07 But that's in essence what we need to figure out
00:25:13.00 in order to understand this process.
00:25:15.07 There's no machinery around,
00:25:16.18 so we have to identify the chemical and physical processes
00:25:20.14 that will drive growth and then mediate cell division.
00:25:25.11 So that applies not only to the membrane,
00:25:27.11 but also the genetic material, whether it's RNA or something else.
00:25:31.13 There have to be simple chemical processes
00:25:35.16 that will drive the copying of that information,
00:25:38.12 that will allow the strands to separate
00:25:41.06 so that another round of copying can take place,
00:25:44.05 and that will allow that replicated material to be distributed into daughter cells.
00:25:50.10 So if we can identify chemical and physical processes that do all of that,
00:25:56.25 we would have a situation where essentially the environment
00:26:00.03 is driving a cycle of growth and division
00:26:04.15 that brings us back to this stage,
00:26:07.05 and you can go around and around that cycle again and again,
00:26:12.05 and that would be just very similar to the way in which
00:26:16.22 modern cells grow and divide.
00:26:18.23 The information within would be propagated and transmitted
00:26:22.23 from generation to generation,
00:26:25.08 and the important thing in terms of the emergence of Darwinian evolution is that,
00:26:31.00 during that continuous process of replication,
00:26:35.07 of course mistakes would be made.
00:26:38.18 Over time, more and more of sequence space would be surveyed,
00:26:43.28 and eventually we think, some sequence would emerge that did something
00:26:48.03 useful for the cell as a whole.
00:26:50.14 As soon as that happened, that sequence,
00:26:54.00 by conveying an advantage to its own cell,
00:26:57.24 whether in terms of growth rate or the efficiency of cell division
00:27:03.00 or the efficiency of survival,
00:27:05.22 it would have an advantage and it would gradually over generations
00:27:08.28 take over the population.
00:27:11.09 And so that is really the essence of Darwinian evolution.
00:27:15.05 You have a change in the genetic structure of the population as a result of natural selection.
00:27:21.01 And that is precisely what we would like to see
00:27:24.03 emerge spontaneously in our laboratory experiments.
00:27:27.17 We want to start with a chemical system
00:27:30.18 and watch it transition into the emergence of real Darwinian
00:27:35.18 evolution at a very simple level.
00:27:39.20 So, let's step back again and think about how all of these
00:27:43.25 molecules would be made in the environment of a primitive planet.
00:27:49.02 And of course, the first breakthrough in this research program
00:27:54.03 was the famous Miller-Urey experiment,
00:27:57.23 in which a mixture of reducing gases was subjected to an electric spark discharge,
00:28:02.10 and the products were analyzed.
00:28:04.12 And amazingly, in that mix of products were many of the amino acids,
00:28:11.19 which are major components of the proteins of modern cells.
00:28:16.28 So that was really a revelation.
00:28:19.29 It really took people by surprise that the building blocks of biological structures
00:28:28.05 could be generated in such an easy manner.
00:28:33.01 Now, in fact that result was so powerful
00:28:36.08 that it might have actually been a little bit distracting.
00:28:40.15 Probably the really important thing that's made
00:28:43.19 in this kind of experiment is not amino acids per se,
00:28:48.03 but high-energy intermediates like cyanide and acetylene.
00:28:53.06 Those are the kinds of molecules that can assemble in
00:28:57.25 subsequent steps into nucleotides, the building blocks of genetic materials.
00:29:07.16 Those molecules are thought to have been made in primitive environments,
00:29:14.11 so that was an electric discharge experiment,
00:29:16.08 which is very analogous to the kinds of lightning displays
00:29:21.12 that you get in volcanic scenarios.
00:29:24.12 So this is the lightning that's going on in the ash cloud
00:29:29.14 of a currently erupting volcano in southern Chile.
00:29:34.02 So since the early Earth was thought to be highly volcanically active,
00:29:38.15 this seems like a very reasonable scenario.
00:29:42.13 What about some of the other molecules that we need
00:29:44.22 to build our primitive early cell?
00:29:50.29 We need to have lipid-like molecules, amphiphilic molecules
00:29:55.29 that can self-assemble into membranes and generate compartments spontaneously.
00:30:00.16 So these are molecules that are amphiphilic:
00:30:03.07 They have one part that likes to be in water,
00:30:05.26 and another part that doesn't like to be in water.
00:30:09.11 And the way that those preferences are balanced
00:30:12.29 is by forming membranes in which the nonpolar parts are on the inside
00:30:17.29 and the polar parts of the molecule face out into the water.
00:30:21.16 So it turns out that it's actually, again,
00:30:24.19 very easy to make molecules like that in a variety of different scenarios.
00:30:30.02 In fact, Dave Deamer and his colleagues showed that you can
00:30:34.08 extract molecules from the Murchison meteorite
00:30:37.11 (it's one of these carbonaceous chondrite meteorites that's rich in organic materials),
00:30:41.23 you can extract molecules that will self-assemble into a vesicle,
00:30:46.10 as you can see here.
00:30:48.04 So they spontaneously make membrane sheets that close up into small vesicles.
00:30:53.19 Here's another example.
00:30:56.00 This is an experiment that was done to
00:31:00.11 mimic processes going on in interstellar molecular clouds,
00:31:05.03 where you have various gasses that have condensed
00:31:07.22 on the surface of silica particles.
00:31:10.11 They're subjected to irradiation by ultraviolet light and ionizing radiation.
00:31:16.03 So if you make ices like that in the laboratory,
00:31:19.18 subject them to ultraviolet radiation,
00:31:22.29 you get a lot of complicated chemistry going on,
00:31:25.01 and then in that vast mix of products,
00:31:28.28 you can extract molecules which again will form membranes
00:31:32.01 and self-assemble into these vesicle compartments.
00:31:37.10 Here is yet another scenario.
00:31:39.03 This is a hydrothermal synthesis done by Bob Hazen and Dave Deamer.
00:31:46.05 Again, in hydrothermal processing,
00:31:50.00 you can grow carbon chains with oxygenated groups
00:31:55.05 such as carboxylates at the end,
00:31:57.05 and these self-assemble into membranes and make many compartments,
00:32:02.03 as you can see in this beautiful image.
00:32:05.29 So, what would be an example of an early Earth
00:32:09.29 environment where something like this could take place?
00:32:14.03 There are a series of experiments from the Simoneit Lab that
00:32:19.14 suggest that hydrothermal synthesis could happen deep down
00:32:24.28 in regions with high temperature and high pressure,
00:32:29.25 on the surface of catalytic minerals such as transition metal sulfides or oxides,
00:32:35.27 and those reactions would basically turn hydrogen and carbon monoxide
00:32:40.27 into fatty acids and related compounds.
00:32:45.05 So the next slide here is a movie that was prepared by Janet Iwasa,
00:32:50.29 that illustrates this process.
00:32:52.15 So we're going deep into the Earth,
00:32:54.23 down through the water channels of a geyser,
00:32:59.00 and here we're looking at the surface of these catalytic transition
00:33:03.28 metal minerals, and you can see hydrogen and carbon monoxide molecules
00:33:08.24 bouncing around the surface, and the mineral is catalyzing
00:33:13.11 their assembly into chains, which eventually will be released and float up,
00:33:21.00 and they'll be caught up in the flow of water
00:33:23.02 and thereby brought to the surface, where you can imagine these fatty acids,
00:33:28.15 fatty alcohols, and related molecules being aerosolized
00:33:32.22 and concentrated in droplets and perhaps even
00:33:35.20 building up into large deposits on the land surface.
00:33:41.05 So it doesn't seem like the prebiotic assembly of molecules
00:33:46.07 that could spontaneously form membrane vesicles is all that difficult.
00:33:53.03 It's definitely an understudied area of prebiotic chemistry, it needs more work,
00:33:57.16 but it looks, I think, reasonably plausible.
00:34:01.13 So the most prebiotically likely molecules
00:34:04.15 would be things like capric acid that you see down here.
00:34:08.24 Short chain, saturated fatty acids.
00:34:12.15 So we do experiments in the lab with molecules like this,
00:34:16.28 but we also use longer chain, unsaturated molecules
00:34:22.07 like myristoleic acid and oleic acid,
00:34:25.05 as model systems because they're just generally easier to work with.
00:34:29.16 So what happens if you just take one of these fatty acids
00:34:34.20 and shake it up in water with some salt and buffer?
00:34:38.06 Is it hard to make membranes? No.
00:34:40.20 What you can see if that you just spontaneously make vesicles
00:34:47.27 in a huge variety of complex structures, a huge range of sizes,
00:34:53.25 all the way from 30 microns (this large vesicle)
00:34:57.19 to many, many smaller vesicles ranging down to 30 nanometers.
00:35:02.29 Many of these vesicles are composed of multiple sheets of membrane,
00:35:08.04 so stacks of membranes.
00:35:10.04 You can see some of these vesicles have smaller vesicles inside them.
00:35:14.17 So it's a very heterogeneous, complex mixture.
00:35:19.29 Now, the other thing that's really important about this is that these vesicles,
00:35:25.25 these membranes, have very, very different properties
00:35:29.02 from modern biological membranes.
00:35:31.26 Modern membranes are basically evolved to be good barriers,
00:35:37.09 so that cells can control the flow of all molecules in and out
00:35:42.04 using complicated protein machines.
00:35:47.26 For a primitive cell, you wouldn't want a situation like that...
00:35:51.03 that would be suicidal.
00:35:52.15 These molecules have to let stuff get across,
00:35:55.03 they have to have dynamic properties
00:35:56.26 that can let them grow and equilibrate.
00:36:00.16 So the next slide is actually a movie, again prepared by Janet Iwasa,
00:36:05.13 to illustrate the dynamic properties of these vesicles,
00:36:09.29 which are so different from modern membranes.
00:36:13.06 And so what you can see here is, first of all,
00:36:15.08 the motion on the surface, a lot of oscillations, diffusion.
00:36:20.27 In the membrane itself, these molecules, the individual molecules
00:36:25.05 are rapidly flip-flopping back and forth from inside to outside,
00:36:29.23 they're constantly entering the membrane, leaving the membrane,
00:36:34.16 so there's a lot of exchange reactions that are
00:36:37.29 going on on very rapid timescales, on the order of a second or less.
00:36:42.23 So they're very dynamic structures.
00:36:44.23 And these dynamic motions are also probably
00:36:49.09 very important in terms of permeability.
00:36:51.26 They allow the formation of transient defects in the membrane,
00:36:55.20 which let molecules get across spontaneously
00:36:58.21 without any complicated machinery.
00:37:02.19 There's another property of these vesicles which I find quite fascinating.
00:37:09.02 So as you saw in the illustration, the molecules that make up
00:37:12.28 any given vesicle come and go and therefore exchange between vesicles
00:37:19.07 on the timescale of roughly a second.
00:37:22.09 In this slide what you see are two populations of vesicles
00:37:25.16 that were labeled with phospholipid dyes,
00:37:28.18 so they're not exchanging between vesicles.
00:37:31.18 The picture here was taken after about a day,
00:37:35.18 and so you can see that they haven't all just fused and mixed up,
00:37:39.01 there are still red vesicles and green vesicles.
00:37:42.11 And yet we know from our other experiments that the molecules
00:37:45.27 that make up any one of these vesicles are changing
00:37:50.29 on a very rapid timescale, yet the structures themselves
00:37:54.23 maintain their identity on the timescale of weeks or months.
00:38:02.03 What about the nucleic acids then?
00:38:05.05 We've talked a lot about the building blocks of membranes,
00:38:07.27 the way they self-assemble,
00:38:09.12 and the properties of the membranes that they assemble into...
00:38:12.27 let's go back to the genetic materials and think about
00:38:15.21 what kinds of building blocks we need to assemble molecules like RNA.
00:38:22.06 Now, again, we have a difference between the molecules used in modern life...
00:38:28.12 so these of course are nucleoside triphosphates,
00:38:32.11 they're almost ideal substrates for a highly evolved cell
00:38:37.13 with very, very powerful catalysts.
00:38:41.15 These molecules are kinetically trapped in a high-energy state.
00:38:47.13 They don't spontaneously act very well at all,
00:38:51.29 so it takes a very sophisticated catalyst to
00:38:54.27 use molecules like this as a substrate.
00:38:57.26 They're also of course very polar, the triphosphate group is highly charged,
00:39:02.16 and that prevents these molecules from leaking out of the cell,
00:39:06.23 which would be a bad thing.
00:39:08.14 On the other hand, in a primitive cell,
00:39:11.19 if you imagine that substrates, food molecules,
00:39:15.05 are being made in chemical processes out in environment,
00:39:19.15 it needs to be possible for those molecules to get across the membrane
00:39:23.17 spontaneously and get into the interior of the cell.
00:39:27.18 So then we to think about different kinds of substrates,
00:39:31.19 molecules that are less polar so they can get into the cell,
00:39:36.25 and more chemically reactive, so that they can polymerize without the need
00:39:43.15 for very sophisticated, advanced, highly evolved catalysts.
00:39:47.24 And so molecules like this were first made by Leslie Orgel
00:39:52.20 and his students and colleagues 20-30 years ago,
00:39:57.13 and studied in quite a bit of detail as models for the early replication of RNA.
00:40:06.17 So, this brings us back to the question of
00:40:12.02 what was the first genetic material?
00:40:14.17 Was it RNA, in fact?
00:40:17.14 Or is RNA so complicated,
00:40:19.24 or its building blocks so hard to make,
00:40:23.03 that life more likely began with something simpler,
00:40:27.18 something easier to make,
00:40:29.12 maybe something more stable that could accumulate,
00:40:31.24 like DNA for example?
00:40:34.16 So this is an area of active debate and investigation,
00:40:39.10 we really don't know the answer to this question,
00:40:43.13 but lots of people are doing experiments and trying
00:40:45.22 to work out chemical pathways leading up to RNA,
00:40:49.09 for example, the Sutherland Lab in the UK has made a lot of progress in this area.
00:40:55.16 We're studying how these molecules could be assembled and replicated.
00:41:01.27 So one of the satisfying thinks about thinking about RNA
00:41:04.22 as the first genetic material,
00:41:06.10 is that we actually have two different chemical physical processes
00:41:12.29 that can lead to the polymerization of activated building block
00:41:17.01 into long RNA chains.
00:41:19.16 The first of these was discovered by Jim Ferris, working with Leslie Orgel,
00:41:26.27 and that was the discovery that a common clay mineral
00:41:30.20 known as montmorillonite can catalyze the assembly of
00:41:34.19 nucleotides into RNA chains.
00:41:37.04 So this illustrates the structure of this clay,
00:41:39.25 it's a layered hydroxide mineral.
00:41:43.03 In between the layer, the aluminum silicate layers,
00:41:47.21 there's water, and in these inner layers,
00:41:50.27 organic molecules can accumulate, and when they're brought close together,
00:41:54.23 they can react each other and start to polymerize.
00:41:58.25 So here is some of the experimental data.
00:42:01.29 So over a period of days, you start off with small chains,
00:42:07.08 and then gradually they get longer and longer, up to lengths of roughly 40,
00:42:12.18 and in more recent experiments up to 50 or 60, nucleotides long.
00:42:17.27 So I wanted to illustrate that with this movie,
00:42:20.16 another one of Janet Iwasa's animations,
00:42:24.16 to show roughly how we think this works.
00:42:27.15 So these chemically activated building blocks like to stick to the
00:42:31.13 surface of the clay mineral,
00:42:33.20 and when they stick in such a way that they're lined up with each other,
00:42:38.05 they can react and assemble a chemically linked backbone,
00:42:43.16 as you see here.
00:42:47.28 Now, there is another process that can do that same thing,
00:42:51.05 which is very interesting because it's so counterintuitive.
00:42:54.05 It turns out if you take these same building blocks and just have them
00:42:57.29 in a dilute solution and put that on your bench, nothing happens.
00:43:03.10 But if you take that same solution and put it in the freezer
00:43:07.08 and then come back the next day, you'll find RNA chains.
00:43:11.20 Why is that?
00:43:13.09 It's because when water freezes and forms ice crystals,
00:43:16.26 that during the growth of the ice crystals,
00:43:19.12 other molecules (solutes) are excluded from the growing crystal,
00:43:23.23 and so they end up concentrated as much as a thousand fold
00:43:27.21 in between the grains of ice,
00:43:31.04 and so when they're so concentrated, again they can react and polymerize.
00:43:35.08 So having two different processes that can lead the assembly of
00:43:38.23 RNA chains is actually a very satisfying thing...
00:43:41.29 that's something we look for in this field,
00:43:43.29 if there's more than one way of solving a problem,
00:43:47.04 it makes the whole solution seem more robust.
00:43:51.15 Now, the hardest problem, perhaps,
00:43:54.07 is once you've got RNA chains like this, how can they be replicated?
00:43:59.16 So much of our early thinking was based on RNA catalysis,
00:44:04.19 and in fact the whole basis of the RNA world is the idea
00:44:07.16 that RNA can act as an enzyme that could catalyze its own replication.
00:44:13.18 And Dave Bartel, when he was a student in my lab many years ago,
00:44:19.17 actually evolved an RNA enzyme with a catalytic activity,
00:44:25.11 that can ligate together pieces of RNA.
00:44:29.04 And Dave subsequently evolved this ribozyme into an even more complex structure
00:44:35.07 that is really an RNA polymerase made out of RNA.
00:44:40.09 Now, that's a very impressive proof of principle,
00:44:43.26 but unfortunately, despite many advances over the years,
00:44:47.26 we're still far from having an RNA molecule that can
00:44:51.09 completely catalyze the copying of its own sequence.
00:44:55.28 So, what we've decided to do is to actually again step back
00:45:01.07 and try to look at the underlying chemistry
00:45:04.03 and see if there might be ways of adjusting or playing
00:45:10.05 with the chemistry of RNA polymerization that would simplify this problem.
00:45:17.04 Ideally, perhaps we will be able to find a complete chemical process
00:45:22.00 that could drive RNA replication.
00:45:25.15 Now, that's a very difficult task,
00:45:28.16 Leslie Orgel and his colleagues worked on that for many years,
00:45:32.17 got partway to a solution,
00:45:34.22 but were never able to have complete cycles of replication.
00:45:39.23 But we have decided to go back and look at some model systems
00:45:44.16 and see if we can get some clues as to how to approach that problem,
00:45:48.24 perhaps in some fresh ways.
00:45:51.13 So, just to illustrate what we're really after,
00:45:53.24 I'm going to show another of Janet Iwasa's movies,
00:45:57.16 and so what you see here is an RNA template, a single-stranded molecule,
00:46:02.00 floating in a solution full of activated monomers,
00:46:05.05 which then find their complementary bases,
00:46:07.26 so they use Watson-Crick base pairing to line up on the template,
00:46:11.06 and then they basically click together to build up a complementary strand,
00:46:16.16 generating a duplex product.
00:46:20.25 So we're after some kind of simple,
00:46:25.03 chemical system that would drive that process very efficiently.
00:46:30.18 So, if we could get to that point,
00:46:33.06 then we would be back to being able to assemble this kind of model system,
00:46:39.10 a model protocell, composed of a membrane compartment boundary
00:46:45.07 and replicating genetic material on the inside.
00:46:50.28 Now, when we're thinking of a complex composite system like this,
00:46:56.13 the question often arises as to,
00:46:58.06 well, why really bother with the membrane compartment?
00:47:01.10 Why not just let the RNA molecules replicate in solution?
00:47:05.17 And one way of thinking about that is that,
00:47:09.22 for Darwinian evolution to emerge,
00:47:12.27 molecules that are in some way better than their neighbors
00:47:15.25 have to have an advantage for themselves.
00:47:19.01 So if we think about RNA replicases floating around in solution,
00:47:24.02 so these would be RNA molecules that catalyze the replication
00:47:27.24 of another RNA molecule,
00:47:30.14 it doesn't really help if you have a mutation which is faster or more accurate,
00:47:37.23 if all it's doing is copying random, other RNAs
00:47:41.01 that it bumps into in solution.
00:47:44.01 It has to have an advantage for itself.
00:47:47.18 And the simplest way to imagine that happening
00:47:50.13 is to encapsulate these molecules within a vesicle,
00:47:54.20 so that they're always copying molecules that are related by descent.
00:48:01.03 Now, the self-assembly of these kinds of complex structures
00:48:06.22 is something that's actually quite simple.
00:48:09.28 So, at the lowest level,
00:48:12.26 the formation of a membrane vesicle can just encapsulate
00:48:16.01 whatever is there in the surrounding solution.
00:48:19.17 However, it's intriguing that there are ways of making the process more efficient,
00:48:24.02 and one of the most interesting ways of doing that is
00:48:28.05 to take advantage of that same clay mineral, montmorillonite,
00:48:31.25 that we've already seen can catalyze the assembly of RNA strands.
00:48:36.24 And so what you can see in this picture,
00:48:39.19 which was generated by Shelly Fujikawa and Martin Hanczyc
00:48:44.11 when they were in my lab about eight years ago...
00:48:47.03 what you can see is that we have here a clay particle,
00:48:51.15 which has RNA molecules bound to its surface,
00:48:55.21 so the orange color is a dye-labeled RNA,
00:48:59.02 and it turns out these clay particles can catalyze the
00:49:01.28 assembly of membrane sheets from fatty acids.
00:49:08.15 And what's happened here is that this clay particle has catalyzed
00:49:11.15 the assembly of this large surrounding vesicle
00:49:15.29 as well as the many smaller vesicles encapsulated within.
00:49:20.03 So what we now can see is that a single very common,
00:49:25.11 abundant mineral can catalyze the assembly of a genetic material,
00:49:30.17 it can catalyze the assembly of compartment boundaries (cell membranes),
00:49:34.08 and it can help bring them together.
00:49:36.13 So very intriguing as a way of simplifying the assembly of
00:49:39.14 cell-like structures on the early Earth.
00:49:43.02 Here's another picture: clay particle inside a vesicle.
00:49:48.01 Here the boundary is quite dramatically evident,
00:49:52.07 so this is a stack of many layers of membrane bilayers.
00:49:56.26 Here's yet another example where the large outer vesicle
00:50:00.15 is filled with hundreds of smaller vesicles, all assembled under
00:50:04.08 the catalytic influence of this clay particle in the middle.
00:50:10.11 So, assembling these things looks fairly simple.
00:50:14.02 What about the process of growth and division?
00:50:16.21 After all, that's what we really need to generate
00:50:19.07 cell-like structures that can propagate.
00:50:22.18 And at this point,
00:50:25.01 what I can say is that we've come up with a process that looks fairly robust.
00:50:31.09 We can start with vesicles and food in the form of fatty acid micelles.
00:50:37.16 They grow remarkably into filamentous structure,
00:50:42.13 which can then divide very easily into daughter cells,
00:50:45.19 and this generates a cycle that can go around and around indefinitely.
00:50:51.08 And in the next part of this lecture,
00:50:53.26 I'll go into much more detail about the nature of this process
00:50:57.09 and the mechanism by which this happens.
00:51:01.02 But, putting this cycle together with our
00:51:06.17 thinking about nucleic acid replication,
00:51:09.13 we can actually start to imagine what a
00:51:12.13 primitive cell cycle would have looked like.
00:51:15.12 And so this is shown in this figure from a Scientific American article
00:51:20.04 that I wrote with Alonso Ricardo from my lab,
00:51:23.03 and it summarizes some of our ideas about the ways in which
00:51:28.28 the early Earth environment might help to drive cell growth and division.
00:51:35.22 So the idea is that the general environment should be rather cold,
00:51:40.15 perhaps even an ice-covered pond,
00:51:44.24 something you might find in an arctic or alpine environment.
00:51:48.29 There are many examples on the modern Earth.
00:51:52.16 The reason for wanting a cold environment in general is that the
00:51:56.08 copying chemistry seems to go better at low temperatures.
00:52:01.06 The low temperature helps the building blocks
00:52:03.10 to bind to the template and facilitates the copying process.
00:52:07.10 But then we know that eventually, once copying is complete,
00:52:10.28 you have to get the strands apart so that you can
00:52:13.09 undergo another round of copying.
00:52:15.21 Simplest way for that to happen is to invoke high temperatures.
00:52:19.23 And so what we like to think about are
00:52:21.21 convection cells driven by geothermal energy;
00:52:26.12 so essentially in a hot spring type of environment,
00:52:30.13 you could have a pond that's mostly cold,
00:52:32.20 but every now and then, these particles would get caught up
00:52:35.01 in a plume of hot water rising from a hot spring.
00:52:39.01 They'd be transiently exposed to high temperatures
00:52:41.24 that would result in strand separation.
00:52:45.02 It also allows for a rapid influx of nutrients from the environment
00:52:50.02 to feed growth and replication through the next round.
00:52:54.08 And then that would generate a cycle in which the
00:52:57.25 entire process of growth and replication and division
00:53:01.07 is driven by fluctuations in the environment.
00:53:04.20 This is driving us to talk to geologists and to search
00:53:09.19 for analogues of this kind of environment on the modern Earth.
00:53:13.13 Here is a beautiful image of an Antarctic lake
00:53:18.19 in which you see stromatolites,
00:53:20.15 these mounds here are microbial growths on the surface,
00:53:25.18 and the reason that it's liquid is of course there is heat rising up from
00:53:29.18 below geothermally, so it's not a perfect analogue
00:53:34.15 of the scenario I described.
00:53:36.05 We'd like to find environments like this where there are hot springs
00:53:39.12 generating convection cells that could drive the whole cycle.
00:53:43.05 So that would be very satisfying if we could identify such environments.
00:53:48.29 So, what I've tried to show in this lecture is basically
00:53:54.13 the context of the environment and the chemistry
00:53:57.10 leading up to the assembly of primitive cells,
00:54:00.25 in a way that's plausible on the early Earth.
00:54:03.17 And what we'll head into in the next two parts
00:54:06.18 are a more detailed look at the chemistry of membrane assembly,
00:54:11.11 growth, and division;
00:54:12.16 and the chemistry of nucleic acid replication.
00:54:15.25 And all of this work is of course has been done through many
00:54:20.17 very talented students and postdocs in the lab
00:54:25.03 who you can see here on this slide.
00:54:28.13 Thank you.

00:00:02.17 Hello, and welcome to iBioSeminars. My name is Dianne Newman, and I'm a professor in the Divisions of Biology
00:00:07.16 and Geology and Planetary Sciences at the California Institute of Technology,
00:00:11.26 and I am also an investigator at the Howard Hughes Medical Institute.
00:00:15.13 So I am going to be giving you a lecture in three parts today,
00:00:19.16 and this is part one, which will be a very general overview on microbial diversity and evolution.
00:00:25.09 In part two, I'll tell a specific story about a modern example of a microbial metabolism
00:00:30.29 that's quite interesting and very important in affecting the geochemistry of the environment with
00:00:36.04 regard to arsenic geochemistry.
00:00:38.06 And in part three, I'll talk about work we've been doing that is more directed
00:00:43.17 at understanding a metabolism that evolved in the past, namely oxygenic photosynthesis.
00:00:49.02 But let's start with an overview now
00:00:52.14 and consider four important points about
00:00:57.00 microorganisms and their history. And I am going to walk you through each of these four points.
00:01:02.14 So the microbial world is really quite remarkable
00:01:06.23 and my goal in this first overview is to leave you with an impression
00:01:10.22 of its diversity, its antiquity, and how abundant and ubiquitous
00:01:16.18 this world is. So let's begin with antiquity.
00:01:20.27 When we think about the evolution of life, oftentimes we think in terms of macroscopic
00:01:25.23 fossils such as the ones that you see here. And it is pretty clear when you look
00:01:30.01 at these rocks, that something living was present on Earth when they formed.
00:01:33.24 In this panel over here you see a fossil of some type of algae. It is not clear
00:01:39.26 exactly what type, but it is inferred to be an alga.
00:01:42.09 And this shape here is known as a trilobite.
00:01:45.06 And this section of rock that you are looking at is one of the most famous fossils on the plane.
00:01:50.12 It is called the Burgess Shale. It's found in Canada, and it dates to what we call the Cambrian explosion,
00:01:57.14 which occurred roughly half a billion years ago around
00:02:00.29 five hundred and sixty million years ago.
00:02:04.16 So we can certainly claim when we look at rocks of this age that life was present.
00:02:09.17 But if we want to think about the evolutionary history of life,
00:02:12.22 over a much larger time span of billions of years,
00:02:15.26 given that the Earth is 4.6 billion years old,
00:02:20.02 we need to step back in time and look at more ancient rocks.
00:02:22.12 And when we do this, the shapes suddenly change, and it becomes
00:02:26.03 not quite as evident that we are looking actually at fossilized versions of life,
00:02:30.09 and yet we are. So for instance, take this rock as an example.
00:02:34.07 Here you see these dome-like structures, and these are vestiges of a type of microbial community
00:02:41.29 forming in a shallow marine environment
00:02:44.16 that became lithified and left these domal structures,
00:02:48.07 and we call these structures stromatolites.
00:02:50.08 Now this particular rock that you are looking at
00:02:52.21 is about 3 billion years old and is from South Africa.
00:02:56.19 But these rocks can be found all over the world, and they occur
00:02:59.21 throughout Earth's history, going back as far as 3.4 billion years.
00:03:04.20 However, when we go even further back in time,
00:03:08.04 for example, back to 3.8 billion years,
00:03:10.17 you can see ore deposits that one might not intuit immediately had anything to do with
00:03:16.26 microorganisms, and yet they do. They indeed record a history of microbial activities that was quite profound,
00:03:24.24 so profound that it quite literally transformed the planet.
00:03:27.19 And this is one beautiful example. So what you are looking at here is actually a 2.4 billion year old quarry.
00:03:35.27 This is in Western Australia in the Hamersley formation, and this is known as a banded iron formation.
00:03:41.13 And they're extremely important today because they constitute the world's largest source of iron ore.
00:03:47.03 But they also record a remarkable history of the evolution of metabolism.
00:03:51.13 Now how can this be? How do these massive rock quarries tell us anything about microbial life?
00:03:57.14 Well, when you think about what they actually constitute
00:04:01.00 they are made up of iron minerals, as well as other minerals
00:04:04.21 cherts, which is a type of silicon oxide,
00:04:07.07 intermixed with these iron species, but for now let's just focus on the iron.
00:04:12.02 So how did this iron get into this big deposit that you see here?
00:04:15.12 Well, it began a long time ago in ancient seas, in the form of ferrous iron
00:04:20.23 that's called Fe2+.
00:04:22.17 And then some process, which I'll get to in just a minute, oxidized this ferrous iron to ferric iron,
00:04:29.00 and at that point it could react with constituents in the waters such as hydroxyl species,
00:04:34.26 to form iron minerals, such as this one: ferric oxyhydroxide, rust.
00:04:41.01 And over time this mineral transformed and changed
00:04:44.19 into different types of minerals, became compacted,
00:04:47.17 and mingled with others and wound up in these rocks that we today know as banded iron formations.
00:04:54.14 But this initial step here is the critical one in terms of giving us some insight into microbial
00:05:00.23 activities on the ancient Earth. And let's think about two scenarios where microorganisms
00:05:06.05 might have been involved. The first scenario
00:05:09.03 is one where a very primitive type of photosynthetic organism,
00:05:12.27 well, I should say primitive in quotes, because actually this metabolism is remarkably sophisticated.
00:05:17.16 Nonetheless, this is primitive in the sense that it is a type of photosynthesis
00:05:21.18 that does not generate oxygen. Rather it is called anoxygenic, meaning that there is
00:05:29.09 an electron donor, in this case ferrous iron, that is oxidized to ferric iron
00:05:35.29 and that powers the reduction of inorganic carbon, CO2, to biomass.
00:05:42.18 And you can see this is a very dramatic metabolism
00:05:46.25 when it occurs because all you need is light, microorganisms, and ferrous iron,
00:05:52.21 and a few other things to help them get going, but those are really the three most important ones
00:05:57.22 in a bottle here, with, as I said, a few nutrients added so they can do their thing,
00:06:04.19 and when light is shined on this bottle,
00:06:08.24 these organisms very rapidly are able to oxidize the iron.
00:06:12.19 And they produce rust, and you can see the rusty color here in this bottle.
00:06:16.14 And this rust is exactly the type of iron that is the predecessor of the minerals that constitute
00:06:22.04 these banded iron formations. Now in the middle you see these organism growing on a different
00:06:27.03 electron donor, and I'll get to what I mean by an electron donor and an electron acceptor later in this lecture.
00:06:33.17 And in this case they are utilizing hydrogen as an electron donor, and the pink color you see
00:06:39.13 is due to photosynthetic pigments in their membranes
00:06:42.16 that enable them to harvest light and grow in this way.
00:06:45.07 So this scenario, as I said, is one that is catalyzed by
00:06:51.01 organisms that do not generate oxygen. They are anoxygenic phototrophs
00:06:55.14 capable of oxidizing iron in a photosynthetically mediated
00:06:58.29 process under environments where no oxygen is present whatsoever, and yet these ferric minerals can form.
00:07:07.22 Now scenario two, that is entirely different
00:07:11.02 is one where the organisms that ultimately catalyze the precipitation
00:07:15.11 of these minerals were producers of molecular oxygen, and these are the cyanobacteria
00:07:21.24 that you can see here that were critically important in the history of the evolution of metabolism
00:07:27.08 and quite frankly also in changing the overall chemistry of the Earth including its atmosphere
00:07:33.04 because they evolved the ability, the remarkable ability, to use water as an electron donor in photosynthesis,
00:07:40.05 oxidize it to molecular oxygen, and through this process, power the reduction of CO2 to biomass.
00:07:48.19 Now once they produce this oxygen, the oxygen chemically would have been able to react
00:07:55.09 with ferrous iron, oxidizing it to ferric iron,
00:07:58.25 and then this in turn would go down the pathway to precipitate these rusty minerals I showed you.
00:08:03.06 So here we have two options: one scenario where no oxygen is involved,
00:08:10.11 and a second scenario where oxygen is mandatory.
00:08:12.21 And both of these are biological processes.
00:08:15.22 So how do we distinguish between them if we are interested in understanding
00:08:20.19 the types of organisms that were present on Earth in the remote past?
00:08:23.29 Well this is quite a challenge, indeed, and there will be many years of investigations in the future
00:08:31.07 in order to really pin this down.
00:08:32.29 And it is a great field to get into if you are a beginning student
00:08:36.07 and interested in both biochemistry and evolution,
00:08:39.16 but what I'll say just for now is that we know from a variety of indicators
00:08:43.23 that somewhere between 2 billion and 3 billion years old
00:08:47.26 it is very probable, indeed it is almost certain,
00:08:52.05 that the process of oxygenic photosynthesis arose.
00:08:54.19 But when exactly this happened and how the evolutionary events came together
00:09:00.19 such that these anoxygenic phototrophs that can utilized reduced substrates
00:09:06.05 such as hydrogen, or sulfur species, or iron as electron donors in photosynthesis
00:09:12.13 morphed into a more sophisticated type of phototroph,
00:09:18.00 that was capable of using water as an electron donor,
00:09:20.25 the cyanobacteria, which in turn, are what became the plastids,
00:09:26.22 the chloroplasts that we find in modern marine algae
00:09:30.10 and also of course, in plants that are very well known for their ability to do oxygenic photosynthesis.
00:09:36.09 We do not know. We do not know when this happened.
00:09:39.14 And in my third lecture in this series, I will discuss ways that we can begin to approach this problem.
00:09:44.22 But it's a profound question, and what I would like to leave you with now
00:09:47.27 is just the simple message that these very ancient rocks, such as these banded iron formations,
00:09:52.16 here are holding clues to a mystery that we have to unravel.
00:09:56.22 And it is through tools of modern biology that ultimately we hope to get there.
00:10:01.13 All right, now as I said the history of microbial life
00:10:06.19 extends very far back in time, as far as 3.8 billion years as we currently estimate,
00:10:12.22 but this might have been even earlier for all we know.
00:10:15.22 How do we decipher when particular microbial metabolisms evolved and what types they were?
00:10:22.04 Well, this indeed is extremely challenging.
00:10:24.09 And there are three primary ways that we can gain insight
00:10:28.04 into the microbiology of the past through using either
00:10:31.25 morphological, molecular, or genomic, which is of course a form of molecular biosignatures.
00:10:38.21 And these are very different in what they can tell us.
00:10:41.20 So the first two, morphological and molecular,
00:10:45.01 are important because they can be concretely linked to rocks, old rocks, that we can date.
00:10:51.02 And because of this when we see a particular form,
00:10:55.09 this is being held in the hand a sample of stromatolite.
00:10:58.29 This is at a very different scale here. You are looking at a thin section of a rock,
00:11:03.14 and that is true for these images below
00:11:05.28 where the scale is about 1 millimeter, in this image, and it is even smaller down here.
00:11:12.10 The structures that you observe have been interpreted as being vestiges of ancient life for various reasons.
00:11:19.13 But this interpretation is often ambiguous, and it is a challenge to be able to come up with unambiguous
00:11:25.06 biosignatures simply on the basis of their shape.
00:11:28.12 And so geobiologists, those interested in seeking to understand life in ancient times,
00:11:35.02 have turned recently to what we call molecular biosignatures that come in two forms:
00:11:39.27 either organic biosignatures, or some type of inorganic biosignature,
00:11:46.26 often expressed as a ratio of different isotopes in a sample.
00:11:51.04 Now this in turn is challenging as well,
00:11:54.25 but it may be the best way that we can gain more specific insight into
00:11:59.18 different types of metabolisms, by looking at actually the chemistry what is left in the rock
00:12:04.10 and being able to deduce through finer scale analyses whether or not
00:12:09.14 this chemistry was one that was uniquely imparted by a biological process.
00:12:13.13 Lastly we can think of genes as fossils, and the genomic record has been crucial
00:12:20.03 in establishing the diversity of life on the planet, as I'll get to in a little while in this lecture,
00:12:25.21 but it also helps us understand the relatedness of different enzymatic functions
00:12:30.22 and how they evolved from one another.
00:12:33.19 While this does not give us a concrete date when these metabolisms
00:12:37.14 evolved, it does provide us with an ability to look at
00:12:41.10 the relationship between different metabolisms, and come up with an order in which they likely were invented.
00:12:49.06 So that is all I am going to say right now on the antiquity of microbial life,
00:12:53.22 and if you are interested, tune in for lecture three in this series,
00:12:56.22 where I will spend some more detail talking about how we use a particular compound found in lipids
00:13:02.13 in modern cells as a potential indicator for oxygenic photosynthesis
00:13:06.19 and whether or not this is a valid thing to do.
00:13:10.01 The next point now I want to turn to is just how numerous microbes are.
00:13:14.22 So let's ask a very simple question. How many microbes are there on Earth?
00:13:18.11 And to bring this into a human reference point, let's begin with the number of the human population.
00:13:23.19 So I am from Los Angeles, which at the latest census,
00:13:27.27 was around 10 million people. And in the state of California, we are up to approximately 35 million,
00:13:34.11 and in the United States in general, nearly 300 million.
00:13:39.16 These are large numbers, but overall in the world we are up three orders of magnitude.
00:13:44.18 at 6 billion people. And that's a lot of folks.
00:13:47.27 However, this is nothing in comparison to the microbial population
00:13:51.18 as estimated by a wonderful paper that I am citing here at the bottom of the slide
00:13:55.22 called, "Prokaryotes, the Unseen Majority" that was published in PNAS in 1998.
00:14:01.29 These are very rough numbers, but give or take an order of magnitude
00:14:05.01 here or there, I think you are going to be impressed when you see the number that I am about to show you.
00:14:09.06 So the estimates for the microbial population are just enormous, 5 times 10 to the 30th cells.
00:14:15.08 And this indeed is such a large number that it is very difficult to wrap our minds around it.
00:14:20.21 So to try to make this a bit easier to do,
00:14:22.16 I did a very simple calculation, where I assumed that the length of a given micro-organism
00:14:27.01 was one micron and asked, "how many times would we need to go back and forth between the Earth
00:14:33.11 and the Sun if we lined up all of these organisms end to end in order to account for this number?"
00:14:38.29 And the answer, shockingly, is we would need to go back and forth 200 trillion times.
00:14:45.12 So hopefully that impresses you with just how many of these creatures there are on the planet.
00:14:49.28 Now where are they, if there are so many?
00:14:52.08 How come we don't think about this all the time?
00:14:54.27 Why aren't we overwhelmed?
00:14:56.04 Well, one reason is that oftentimes we are shockingly ignorant
00:14:59.19 about the fact that they are all around us, that we ourselves are walking micro-organisms.
00:15:04.04 So one of the first scientists to appreciate this profound fact
00:15:08.10 was the father of microscopy, Antony van Leeuwenhoek.
00:15:11.11 And this is a lovely image that he drew from his observations down his first microscope
00:15:18.01 in 1684, and you can see he drew some nice rods and cocci, and even pictures of probably motility
00:15:26.22 what is meant by these dotted lines from C to D.
00:15:30.29 And he reflected, as he was looking through the microscope about his own teeth, and this is I think a very funny quote.
00:15:37.28 He said, "Though my teeth are kept usually very clean,
00:15:40.24 nevertheless when I view them in a magnifying glass,
00:15:43.25 I find growing between them white matter as thick as a wetted flower.
00:15:47.09 The number of these animals in the scurf of a man's teeth,
00:15:50.05 are so many that I believe they exceed the number of men in a kingdom."
00:15:54.17 Well, this indeed is actually an underestimate.
00:15:58.28 Not only do they exceed the number of men and women in a kingdom,
00:16:02.13 they go far beyond that. So if we actually look at our own bodies...
00:16:06.13 just take a look at your wrist, at one square inch on the surface of your wrist.
00:16:10.23 Right there, we are estimated to have five to fifty thousand bacterial cells.
00:16:16.22 And it just increases in density as we move to other parts of the body, such as the groin and the underarms,
00:16:22.28 in our teeth, and really where it's mainly at in our bodies is in our colon.
00:16:29.06 And the overall total per person is seventy trillion.
00:16:33.25 That is quite a lot.
00:16:35.24 And one thing that I think is really important for you
00:16:38.18 to know about the microbial community within your own body,
00:16:42.16 is that there are ten times the number of microbial cells
00:16:47.04 in our system than there are human cells.
00:16:51.21 And not only that, when we look at the genetic potential of the DNA
00:16:56.09 within these organisms, the genetic potential of only those within our guts
00:17:02.15 is over one hundred times that of the human genome.
00:17:06.21 So you might begin to ask whether or not humans are not merely walking vats of microorganisms,
00:17:11.29 carriers serving their existence.
00:17:14.08 It is something to think about, and there's a great deal of research now emerging
00:17:18.20 that is beginning to illuminate just how crucial these organisms are
00:17:21.25 for human health, not only with regard to being able to help us digest our food,
00:17:27.07 but also interfacing and controlling our immune system,
00:17:30.09 in ways that are fascinating and profound.
00:17:32.29 Now despite the fact that this number, ten to the twelfth, seems really large, and indeed it is,
00:17:38.25 it's peanuts when we compare it to other domains where we find microorganisms.
00:17:44.09 So let's start with the least abundant, up in the air,
00:17:46.24 It is quite amazing to me that they've been detected as high as thirty four to forty six miles up
00:17:53.06 into the sky. But these concentrations are really small relative to other compartments.
00:18:00.00 As I told you, within the human body we have quite a few.
00:18:04.12 And when you add up all of the humans and domestic animals, and then termites,
00:18:08.08 which I'll get back to in just a bit,
00:18:09.20 the order of magnitude jumps up to about ten to the 23rd, to 24th
00:18:14.06 This is superceded by the quantities that you can find in soils,
00:18:20.06 in forests and grasslands, deserts, tundras, swamp environments.
00:18:24.10 These places are very fertile homes for microorganisms and there their activities can transform
00:18:29.23 the chemistry of their environment quite profoundly.
00:18:31.29 And this is of course also true in aquatic domains, where at similar orders of magnitude
00:18:38.08 we find microorganisms in both marine and freshwater environments.
00:18:42.01 But all of these numbers pale in comparison to the numbers that we find in the subsurface,
00:18:46.14 both in terrestrial and oceanic environments, where microorganisms
00:18:51.09 have been detected as deep as two miles.
00:18:54.22 Now, this really is a very interesting frontier area in microbiology
00:18:58.05 It is hard to go down into these depths, and yet nowadays, researchers are equipped with the tools
00:19:05.02 they need in order to access these remote communities.
00:19:07.26 And what remains to be learned is what exactly these organisms are doing in situ.
00:19:12.11 Are they active? And if so, what are their activities?
00:19:15.26 Are these activities affecting in a significant way
00:19:19.05 the physical and chemical properties of these environments?
00:19:22.07 We don't know, and we look forward in the coming decades to finding
00:19:25.04 the answers to these and other interesting questions.
00:19:27.19 So now let me just give you an example, a tour through various parts of the world
00:19:34.05 and other inhabitants of that world where we find these organisms.
00:19:39.17 Just to bring home to you how ubiquitous microbes are on the planet.
00:19:44.07 So to start with what might be a more familiar image, here what you are looking at is pond scum.
00:19:51.09 You are looking at a wonderful assemblage of phototrophs
00:19:54.27 and other microorganisms in this pond. And my favorites of course are these purple phototrophs.
00:20:00.03 These are the ones that I told you about earlier that are what we call the anoxygenic phototrophs
00:20:04.27 that are not utilizing water as a substrate in photosynthesis,
00:20:08.29 but are utilizing other more reduced compounds such as different types of sulfur species, hydrogen, or iron.
00:20:17.15 Now these organisms that we see in modern day ponds, as I told you at the beginning when I was illustrating
00:20:23.20 the antiquity of microbial life with the example of the banded iron formations,
00:20:27.25 are absolutely historically important for their metabolism,
00:20:33.19 and the diversity of their metabolism, and how it's changed the geochemistry of the Earth.
00:20:38.00 Not only has the evolution of photosynthesis contributed to evolving our atmosphere
00:20:43.20 to one that contains oxygen over the course of time,
00:20:46.14 but as I also showed you with the banded iron formations,
00:20:48.29 these types of organisms have likely shaped ore formation as well.
00:20:54.21 And many other important processes have been able to come about
00:21:00.21 thanks to these organisms doing what they do, and it should be noted
00:21:05.15 that this type of metabolic activity, photosynthesis, is one that today we are highly interested in
00:21:12.06 because of our need for coming up with alternative energy sources, and
00:21:16.25 certainly if chemists were able to mimic what these wonderful microbes in this pond do, we would
00:21:22.25 be able to not worry so much about our dependence on foreign oil
00:21:27.02 and our fossil fuel supplies being burnt, but that's a story for another day.
00:21:32.08 The point is, their metabolic diversity is old. We see it all around us, and the biochemistry is really quite fascinating.
00:21:39.18 Continuing on with the chemistry and the metabolism of these organisms
00:21:44.28 not only do they do important things when they are growing,
00:21:48.00 but they also do important things when they start hitting what we call stationary phase.
00:21:51.24 And this is a point in their development where they're not necessarily actively growing,
00:21:56.12 but they are at a higher density and they are just hanging out metabolically.
00:22:00.26 And when this occurs in their lifecycle,
00:22:04.09 sometimes metabolites and pigments begin to be excreted.
00:22:08.08 And these pigments, which are called secondary metabolites,
00:22:12.14 although that name itself may be a bit misleading, because they are only secondary in a temporal sense,
00:22:18.14 in that they are made after a phase of active growth,
00:22:21.21 but by no means are they secondary in terms of the physiology of the organisms that produce them.
00:22:27.00 None the less, these metabolites oftentimes are used today by pharmaceutical companies
00:22:32.14 as natural products that confer antibiotic activity.
00:22:35.13 And a terrific example of this are organisms in the Streptomycetes family that you see here
00:22:40.18 in this Petri dish that are producing a whole host of wonderful antibiotic compounds.
00:22:45.02 Now containing in the environment of the soil of course are roots of plants.
00:22:51.00 And in this part of soil known as the rhizosphere,
00:22:55.04 we can find microorganisms as well that are colonizing in a very beneficial way the plant roots.
00:23:00.28 And here is a tomato root seedling. This is an image taken by Guido Bloemberg.
00:23:06.25 And he showed in experiments in the laboratory that when he took tomato root seedlings
00:23:12.27 and mixed them with an organism called Pseudomonas,
00:23:16.01 that this bacterium was able to colonize the plant and form what we call biofilms on the surface of the root.
00:23:22.18 And this is just one example of organisms that interact with plants.
00:23:26.20 There are many that fall into this category with different names.
00:23:30.12 And the bottom line is that they have a very beneficial relationship with these plants,
00:23:34.11 where sometimes they produce natural products that fend the plant off from fungal predators
00:23:40.05 and so they serve as biocontrol agents.
00:23:42.24 Other times these organisms are capable of fixing molecular nitrogen
00:23:48.14 into a usable form and essentially acting as a natural fertilizer.
00:23:53.14 Now crawling around in not only soil environments,
00:23:58.00 but of course we are very familiar with these from our homes, are termites.
00:24:01.00 And the termites are a terrific source of microbial diversity
00:24:05.17 and one that is becoming an increasingly important micro-environment
00:24:10.07 in which to look because of our desire to understand microbial processes
00:24:16.00 that might be harnessed for lignocellulose degradation.
00:24:19.04 Again, out of a need to develop alternative sources of energy.
00:24:22.22 Now a colleague of mine, Professor Jared Leadbetter at Caltech
00:24:26.14 studies these termites, and he likes to call them "an ecosystem in a microliter".
00:24:31.05 And I think this is really a fantastic description of them because it is within their hindgut that you find a zoo
00:24:38.21 of microorganisms and protozoa that are swimming around
00:24:43.12 doing all sorts of important activities that make it possible for the termites
00:24:47.12 to digest their wood. And in the process they emit methane,
00:24:52.02 and not an insignificant fraction of this methane ultimately makes its way
00:24:55.16 up into the atmosphere and contributes to the overall chemistry on the planet.
00:25:02.17 So speaking of methanogens, here you see
00:25:05.08 a dramatic illustration of them at work.
00:25:07.18 This image that I am standing in front of is taken from Cedar Swamp in Woods Hole, Massachusetts.
00:25:12.15 And it is an image from a group of students from the microbial diversity class,
00:25:18.02 which is a fantastic course for about twenty students, half from the United States and half from overseas,
00:25:24.07 who come together every summer to understand how
00:25:27.00 microorganisms are able to perform these various metabolic activities
00:25:32.08 that I have been describing in these lectures.
00:25:34.26 And what you can see here is that the students have gone waist deep into this swamp
00:25:39.19 and they have stomped around, and as they have done this they have collected the bubbles that come up
00:25:46.03 as they stomp the sediment, and collected them in these inverted funnels.
00:25:51.11 And then some brave individual holds that funnel
00:25:54.14 and removes their hand just at the moment when a friend comes by with a flame,
00:25:59.09 and ignites it, and here you see a lovely illustration of methane at work.
00:26:04.25 So methanogenesis led to the creation of the methane gas that was ignited here.
00:26:10.06 Now in the past the activity of these organisms that generate this methane
00:26:15.23 that are called methanogens, might have been important in shaping the chemistry of the Earth's environment.
00:26:19.27 And the reason we suspect this may be the case is because early in Earth's history the environment
00:26:25.00 contained appreciably more methane than it does today.
00:26:28.18 Now a different example of a habitat where microorganisms are very important
00:26:33.21 is in Chile and in other places on Earth,
00:26:37.19 but this example here is taken from the Andina Copper mine in the Andes in Chile
00:26:44.23 where microorganisms are exploited for their abilities to help
00:26:48.27 with bioleaching. And so what happens is that in these mines there are piles
00:26:54.06 that are built up, and they are fertilized essentially with indigenous microbial populations
00:27:01.08 that are able to live in shockingly low pH levels,
00:27:05.17 down to pH as low as one, and sometimes even lower.
00:27:08.23 And these organisms are essentially eating the minerals in this mine pile
00:27:14.14 and the process of metabolizing it, changing the mineralogy, in such a way
00:27:19.06 that copper is solubilized and leached. So here is another example of an environment
00:27:25.27 that is quite extraordinary and yet microorganisms have been able to adapt and even to thrive in this extreme condition.
00:27:33.04 So on our tour of extreme pHs, we just saw an example of low pH, so let's go to a high pH environment.
00:27:39.24 This one I am showing you is Mono Lake that is in Northern California.
00:27:44.09 And Mono Lake is quite an extraordinary place. It looks almost like it is from another planet.
00:27:49.25 You see these beautiful tufa towers that are calcium carbonate minerals forming,
00:27:55.05 and it is because the pH is so high and the alkalinity is so high that they naturally precipitate from these waters.
00:28:01.19 In addition to having these carbonate minerals, contained within this lake environment
00:28:07.09 is a ton of arsenic, and I will get to this in part two of my lecture today.
00:28:11.07 And what I want to point out right now is that in this very high pH environment, and also one
00:28:17.14 that's replete with arsenic, nevertheless we find organisms called alkaliphiles
00:28:21.23 that thrive here, that are able to make a living utilizing arsenic as a terminal electron receptor in respiration.
00:28:30.05 This is the subject of my second lecture. And in so doing account for 14% of the carbon turnover in this system.
00:28:38.23 Now let's go on to another example of an extreme environment.
00:28:42.14 Here now we are looking at an extreme of salt.
00:28:45.26 And there is no better example of this than the Dead Sea in Israel, but
00:28:50.06 you can find organisms such as those that inhabit the Dead Sea also in the Great Salt Lake,
00:28:56.04 and other places on Earth such as salt flats, where you have very high salt content.
00:29:04.19 And the organisms living here are capable of growing despite this high salt
00:29:08.21 and have adapted particular molecular strategies to cope with it.
00:29:12.15 One very elegant example of this is their ability to use special photopigments called rhodopsin
00:29:18.23 and these are colored purple.
00:29:21.24 And these rhodopsins, they have in their membranes, and enable them to generate energy
00:29:27.11 under conditions where they need to use slightly different
00:29:30.02 strategies than organisms that are growing under
00:29:33.28 conditions that we would consider more normal.
00:29:36.27 Now, so approaching the end of our tour through microbial diversity and ubiquity, I want to end with a few other extremes
00:29:44.00 now that are based on temperature and pressure.
00:29:46.04 If we think about the extremes of cold there is no better place to go than Antarctica.
00:29:50.27 And you might be surprised to realize that even in this environment you have microorganisms thriving in the crust.
00:29:57.02 And these organisms are psychrophiles, and they're ability to grow is dependent upon dust
00:30:02.26 from winds carrying nutrients picked up from the continents surrounding Antarctica, South America, Australia, Africa,
00:30:14.24 that reach Antarctica, deposit their dust and fertilize these upper crusts of the ice
00:30:20.14 where we have intrepid pioneer organisms that are able to utilize these nutrients
00:30:26.03 and grow, even in these very cold regimes.
00:30:28.02 So another extreme is that of temperature and pressure, and there is no better environment in which to observe this
00:30:34.24 than at the bottom of the ocean, in environments where we have hydrothermal vents
00:30:39.21 that are releasing nutrients into the deep. And here is an example of one of these vents. It is called a black smoker
00:30:46.17 because the nutrients that it releases, including manganese and iron,
00:30:51.14 often precipitate in the conditions of the oceans at these sites
00:30:57.16 such that they look black.
00:30:59.22 Now around these vents there is abundant life,
00:31:04.07 really extraordinary life, not just microbial life.
00:31:06.08 but giant tube worms, and fish, and other macroscopic organisms.
00:31:10.10 So the ability of all of this abundant life to be in this environment
00:31:15.10 crucially depends upon the activities of microorganisms that are chemosynthetic.
00:31:20.01 that are able to grow by the oxidation of sulfur and other compounds that you have
00:31:25.03 present in this environment, and couple that oxidation of these reduced substrates to
00:31:31.05 the fixation of CO2 into biomass.
00:31:34.15 And this is at the base of the food chain that then sustains the growth of other marine organisms.
00:31:40.12 such as these tube worms. And here you see an example of that
00:31:43.10 in these beautiful tubeworms. If you cut them open and you look at one of their organs,
00:31:50.29 called the trophosome within these organs are bacterial symbionts
00:31:55.27 that are doing the process that I just mentioned.
00:31:58.08 So my final example that I will end with
00:32:01.12 is one that might be the most familiar to you
00:32:04.02 if you have ever done any PCR in molecular biology.
00:32:06.25 So most of you have heard of the enzyme Taq polymerase,
00:32:10.06 and this polymerase is what allows us to do an amplification reaction when we are doing PCR.
00:32:18.21 Now this enzyme, Taq, derives from a bacterium called Thermus aquaticus,
00:32:24.28 that is where the Taq comes from. The "T" is from the Thermus and the "aq" from aquaticus.
00:32:31.05 And this is a thermophile that was isolated in Yellowstone
00:32:35.14 at a hot spring, many decades ago.
00:32:38.03 And it was presciently realized by Kary Mullis
00:32:41.18 and others that the enzymes contained within it could be useful for various biotechnological applications
00:32:47.26 because they wouldn't denature at the temperatures that would kill most other types of cells.
00:32:54.02 So these thermophiles are a very fascinating group of organisms
00:32:57.28 whose molecular adaptations include not only DNA polymerases,
00:33:03.26 but also a wide variety of other enzymes that might be of industrial use.
00:33:10.01 So let's now end with diversity, which is really my favorite part
00:33:14.14 of the microbial world. And I want to cover a few different areas of this.
00:33:18.18 The first is phylogenetic diversity.
00:33:20.03 Now one of the most important lessons to be learned in evolutionary theory
00:33:25.03 was learned several decades ago from work by Carl Woese and his colleagues,
00:33:30.03 including Norman Pace, who applied Carl Woese's fundamental
00:33:34.07 insights into the diversity of life to the natural world.
00:33:38.02 And these individuals together with others were able to demonstrate very clearly
00:33:44.05 that when we think about the diversity of life out there on the planet,
00:33:48.16 we are really talking about a microbial world, whether we call these
00:33:52.22 microorganisms Bacteria or Archaea or even Eucaryotes.
00:33:58.23 What I want you to appreciate is that when you look at the tree of life,
00:34:03.11 that's what this is. It is a tree that is drawn based upon comparing the sequences of a very particular molecule
00:34:11.06 that every living organism has, that is ribosomal RNA, that is necessary for the process of translating
00:34:19.07 messenger RNA into protein.
00:34:21.07 Because this is a very universal and highly conserved molecule
00:34:24.28 Carl Woese and colleagues were able to deduce that it was a beautiful molecular chronometer
00:34:31.01 that we can employ to look at the evolutionary relatedness between different organisms.
00:34:36.16 And when he and his colleagues did this , he recognized that
00:34:39.19 there were three primary domains of life, the Bacteria, the Archaea, and the Eucarya.
00:34:43.16 And moreover, what I want to stress now is that our entire universe of Homo sapiens
00:34:52.12 and humans and plants and animals, the macroscopic eucaryotic world,
00:34:57.12 is only occupying in terms of this space on the tree,
00:35:01.27 which is known as a phylogenetic tree, meaning a tree of evolutionary distances
00:35:06.21 between different types of life forms, a very tiny miniscule branch.
00:35:12.00 And everything else that I am showing here is microbial.
00:35:14.13 So hopefully that impresses you, but before we leave this tree let me point out two more
00:35:19.20 facts that are very important.
00:35:20.29 All of the metabolism on the planet was invented by microorganisms
00:35:26.05 including the metabolism that we perform in our bodies today in our mitochondria.
00:35:31.11 So the mitochondrion is nothing more than an ancient bacterial cell
00:35:36.28 that invented the ability to do oxidative phosphorylation, which I'll tell you about in a little bit,
00:35:42.24 that was engulfed or brought into symbiosis with some other type of cell,
00:35:47.23 and over the course of time involved into the organelle that we call the mitochondrion.
00:35:52.28 But it was a microorganism first, and that is where the beautiful metabolism that it goes through was generated.
00:36:02.14 The same story is true for the chloroplast. This is nothing more
00:36:06.03 than cyanobacteria that over time turned into plastids and became incorporated into other cells.
00:36:11.25 Now the next important point I want to make is that microbial diversity also manifests itself morphologically.
00:36:18.19 And this is something that only recently we are coming to appreciate in its full glory.
00:36:24.02 Back in the days of Leeuwenhoek, when he had a simple microscope, all he could really see were different shapes of microbes,
00:36:30.24 and to be quite honest, that is not terribly spectacular and includes rods and spirals and some cocci.
00:36:37.11 Once in a while you see higher structures forming of communities however,
00:36:41.25 and Leeuwenhouk didn't necessarily know about these,
00:36:44.05 but here is an example of one here. This is a beautiful example
00:36:48.13 of fruiting bodies beginning to form by the soil organism Myxobacteria
00:36:53.10 that does all sorts of interesting things when it comes together in a group
00:36:57.01 that it wouldn't do as any individual cell.
00:37:00.25 This is social behavior. So this is an example of bacteria acting in a multicellular fashion, if you will.
00:37:06.11 Microorganisms, however, can get remarkably large. They are not just on the scale of microns.
00:37:11.21 And here is a good example of this.
00:37:13.20 This is, to my knowledge, one of the largest microbial
00:37:16.28 cells known to date. It is called Thiomargarita namibiensis,
00:37:19.29 which means the sulfur pearl of Namibia.
00:37:22.17 And it is on the same scale as the eye of a fruit fly.
00:37:25.13 And when you look at it in more detail, the reason it is so big is that it contains this huge vacuole
00:37:31.21 filled inside with nitrate, which is one of the substrates it uses to power its metabolism.
00:37:38.02 And it couples the reduction of nitrate to a more reduced form of nitrogen
00:37:43.07 to the oxidation of sulfide, and in this way it powers energy for growth.
00:37:48.00 But let's leave the metabolism aside and stay focused now just on the form.
00:37:52.03 Here is an example of one of my favorite organisms, Rhodopseudonomas palustris,
00:37:57.02 and the reason I am showing you this is simply to illustrate
00:37:59.24 that it has quite an amazing membrane structure within it.
00:38:05.27 One that is reminiscent even of the Golgi in higher organisms.
00:38:09.09 And indeed it might have been the progenitor of that at the cell biological level.
00:38:14.17 And how these various structures form, these are what we call
00:38:18.06 the inner cytoplasmic membranes where the photosynthetic machinery is housed in this case,
00:38:22.19 in terms of the detail of what creates their shape is an open and exciting question
00:38:27.08 that future microbial cell biologists will no doubt solve.
00:38:30.17 But the final example, which is probably my all time favorite, is of an organism called a magnetotactic bacterium.
00:38:38.16 And here you see if you just look at it in a light microscope,
00:38:41.15 although this is actually an image of fluorescence where we have put some GFP into the bug,
00:38:46.04 it looks like just a common spiral.
00:38:48.09 If you take a fancier microscope, a transmission electron micrograph, and cut it
00:38:53.01 open and do a thin section, you can see that it has this beautiful chain of magnetic particles inside it.
00:39:00.00 And now what I am going to show you is, I think, the best advertisement for the beauty of bacterial cell biology
00:39:05.28 that I know, and it is a cryo-electron tomogram of one cell.
00:39:13.10 And this was work done by Arash Komeili who is now a professor at UC Berkeley
00:39:18.26 and his collaborator Zhuo Li in Grant Jensen's lab at Caltech. And together
00:39:23.17 we made this movie showing the internal structure of these organisms.
00:39:27.26 So what you are going to see now is coming up through the bacteria different sections,
00:39:32.11 and here you see the magnetosomes coming into view. Those are the membranes that contain the magnetite.
00:39:39.10 If you missed them, now look, OK.
00:39:41.02 Here they are in red, those magnetosome membranes, and then there is this yellow filament surrounding them.
00:39:46.27 And what we have come to appreciate is that this filament is a protein that is very similar to actin.
00:39:52.11 And it is necessary for these magnetosomes, for these organelle-like,
00:39:58.01 although they never separate from the membrane, so they are not true organelles.
00:40:01.14 Here you see they're attached by a neck that's only 5 nanometers in diameter, which is quite amazing, to this inner membrane.
00:40:10.24 They invaginate and form these vesicles within which a beautiful single domain crystal of magnetite can form.
00:40:19.19 And this order, the fact that they are linear in a chain,
00:40:22.21 is enabled by a cytoskeletal filament, an actin-like protein.
00:40:28.17 OK. So the next to the last point that I want to make on diversity
00:40:33.09 is behavioral diversity, and there is another lecture in this iBioSeminar series
00:40:37.06 by Professor Bonnie Bassler from Princeton that can give you more information about this if you are interested.
00:40:41.28 But what I wanted to point out here while we are going through a tour through diversity
00:40:46.13 is simply that microorganisms can act in ways that are quite extraordinary
00:40:51.15 when they are acting as a group.
00:40:53.11 And you can see that illustrated by the activities of the bacterium Vibrio fischeri within the light organ of a squid.
00:41:01.10 And here is an image that is from the beautiful pioneering work of Margaret Mefal-Ngai
00:41:06.18 and Ned Ruby at the University of Wisconsin, Madison where they have been studying for decades the interactions
00:41:12.01 between the microorganisms in the light organ of the squid and the squid,
00:41:16.28 and the ability of these organisms to colonize this environment,
00:41:20.06 and when the lights go out at night, emit a beautiful luminescence. Here you can see pictures of these organisms
00:41:28.12 that have just been streaked out on a plate in the dark.
00:41:30.11 They are glowing. Well, they glow as well here at night in the belly of the squid.
00:41:35.27 And it shields these squids from predators below
00:41:39.00 because the light of the moonlight coming down from the top
00:41:41.28 is roughly of the same luminescence as the light that they are emitting.
00:41:45.26 So it allows them to have a stealth function and glide around in the oceans
00:41:50.19 and be unseen to predators deeper below them.
00:41:56.07 Now, this isn't just a phenomenon that affects the squid. This is a phenomenon that can get quite enormous in its scope.
00:42:03.19 And the best example to illustrate this is this satellite image here taken off of the Somalian coast,
00:42:09.11 where you see an image that quite literally is of milky seas as described by the ancient mariners,
00:42:17.21 but what today we understand as glowing bacteria.
00:42:21.26 In this case an organism, likely called Vibrio harveyi, associating with micro-algae in this environment
00:42:28.22 that for whatever reasons that are not fully understood at this particular point in time
00:42:33.22 when this satellite image was taken, had a bloom and began luminescing like crazy, and filled up a volume the size of Connecticut.
00:42:41.27 All right, so, to end I want to just mention a few rules of microbial diversity
00:42:47.23 because almost everything that I have talked about so far
00:42:51.11 in this lecture ultimately comes back to the ability of organisms to generate energy in ways that are quite amazing.
00:42:59.06 And I would stipulate that microbes are by far the best chemists on the planet.
00:43:02.25 And so if you are a chemist, pay attention, because a lot of lessons can be learned from these guys.
00:43:07.06 All right. Now when we are talking about the phase of active growth,
00:43:11.13 the bottom-line that microbes are facing is they simply want to divide.
00:43:15.12 And to do this they need two things. They need energy, and they need carbon.
00:43:20.08 And beyond that, they are virtually unconstrained, although there are a few constraints,
00:43:26.25 and we will come back to that in a moment.
00:43:29.03 They need substrates, and these substrates can be organic or inorganic compounds.
00:43:36.11 This is now for the part where they are going to be generating energy.
00:43:40.22 Those substrates are converted to products through catabolic reactions, or energy generation, if you will.
00:43:49.09 And often times we think of energy generation in the form of ATP,
00:43:52.20 the most important energy carrying molecule within the cell.
00:43:57.06 Now this part of metabolism, catabolism, is coupled to anabolism,
00:44:03.12 which is the part of metabolism that is concerned with energy consumption, or biosynthesis.
00:44:08.12 And now down here what we are talking about is the conversion of
00:44:12.01 carbon, often in the monomeric form, to biomolecules that are far more complex, so protein, DNA, lipid, for example.
00:44:22.27 Now if we are thinking just about the substrates, as I said they can come from a variety of sources.
00:44:30.06 Always they're chemical, although light can help enable cells
00:44:34.21 to actually utilize those chemicals in ways that they otherwise wouldn't be able to do.
00:44:39.10 But when we are talking about the growth of organisms just purely on chemicals, without needing
00:44:46.15 a boost from light, the name we give to this metabolism is chemotrophy.
00:44:50.18 And that in turn is classified into two different types, inorganic and organic.
00:44:57.06 And when we are talking about inorganic sources of energy like hydrogen, and sulfide, and iron minerals,
00:45:03.18 this is called chemolithotrophy. And when we are talking about growing on organic substrates like glucose, or glycerol, or acetate,
00:45:11.23 this is called chemoorganotrophy.
00:45:14.04 And of course, as I said, while chemistry is always at the basis for any type of metabolism,
00:45:20.20 there is a photochemical boost that is often necessary,
00:45:25.09 when activating a compound that otherwise might not be biologically utilizable
00:45:30.16 for energy, and that is when we call that process a phototrophic one.
00:45:36.10 So the final part of this that I want to just mention is that the carbon source,
00:45:42.02 which is distinct or can be distinct from the energy source...
00:45:44.28 sometimes they are the same thing, but they don't have to be the same thing...
00:45:47.26 is either coming from inorganic carbon, CO2, or organic carbon.
00:45:52.19 And when it is coming from inorganic carbon that is called autotrophy,
00:45:55.21 and when it is coming from organic carbon, that is called heterotrophy.
00:45:58.22 So we are heterotrophs, we need to eat some type of organic carbon whether we are vegetarians
00:46:04.28 or meat eaters, but microorganisms are far more sophisticated.
00:46:09.12 They can eat minerals. They can just take CO2 from the air, and they'll be on their way.
00:46:17.14 So finally the last part I want to mention about metabolic diversity writ quite large
00:46:22.29 is that you can generate ATP through one of two different ways.
00:46:26.13 The first way is through what is called substrate level phosphorylation.
00:46:29.15 And this is also termed fermentation, and essentially is the process
00:46:33.22 where different types of reactions between chemicals within a cell
00:46:39.26 enable transfer of an inorganic phosphate ultimately to ADP to produce ATP.
00:46:49.21 And this process is enabled by chemical rearrangements within the cell
00:46:55.05 and reactions one on one between compounds.
00:46:57.18 The next major way that ATP can be formed in a cell is through the remarkable process of oxidative phosphorylation.
00:47:03.10 Basically this is about electron transport chains in membranes
00:47:07.23 that are coupled to generating a battery around a membrane
00:47:11.08 by extruding protons to one side and polarizing it so that there is an electrochemical potential gradient
00:47:18.00 across this membrane that can be harnessed to do the work of making ATP.
00:47:22.06 Now something that I am not showing you on this diagram,
00:47:25.03 but I want to introduce as terms are an electron donor and an electron acceptor.
00:47:31.00 So in metabolism there is always a substrate that is used as the primary electron donor
00:47:36.14 that can be metabolized through various pathways
00:47:39.08 and reduced to a compound that can donate electrons
00:47:43.17 to the electron transport chain in the membrane.
00:47:45.20 And then there is always something that serves as the acceptor of those electrons at the end of the chain,
00:47:52.10 and that is called the terminal electron acceptor.
00:47:54.28 And it is the path of electron transfer and proton translocation
00:47:58.22 between this electron donor and this terminal electron acceptor that is really harnessed by the membrane to do work.
00:48:05.05 And so that is what you see here pictured very generically without a whole lot of detail
00:48:10.08 in the sense that through this electron transport process,
00:48:13.20 which imagine if you will, is coupled as I said to proton translocation,
00:48:18.00 and that is achieved by different things within this membrane.
00:48:22.15 They can be proteins, or small molecules that are able to simultaneously
00:48:26.25 pass electrons through the membrane to something else
00:48:29.24 in the electron transport chain and push protons, or translocate protons
00:48:34.15 across the membrane so that there is this gradient that arises where there is more positive charge on the outside
00:48:42.07 than on the inside. Now once this happens this gradient can be used to drive ATP synthesis.
00:48:49.21 And this happens through a really amazing molecular machine called the ATP synthase,
00:48:54.09 which allows the traversal through the membrane of a proton,
00:48:58.27 that concomitantly gives the energy to phosphorylate ADP, adding that inorganic phosphate on, and making ATP.
00:49:09.08 And as this happens, the electrochemical potential gradient lessens.
00:49:15.00 And so that is what I am showing here: the energized membrane
00:49:16.25 due to proton transport coupled to electron transfer through the membrane,
00:49:22.10 and then this being expended and used in order to drive ATP synthesis.
00:49:26.12 Now while you can imagine a whole variety of things that can be electron donors
00:49:32.02 and electron acceptors from microbial metabolism,
00:49:34.27 metabolic diversity does have to conform to some rules.
00:49:37.06 And there are three that I want to point out that I think are particularly important.
00:49:41.02 The first is that the amount of energy has to be at a very minimal level,
00:49:47.14 at least in order to sustain the cell, both with regard to active growth, where you need a high level of energy,
00:49:53.27 at some threshold amount in order to double, but also at the level where you are generating enough energy simply to maintain
00:50:01.18 basic cellular processes even if they are not coupled directly to growth.
00:50:05.11 Now what is this number and how do we constrain it?
00:50:09.13 Thermodynamically, this can be expressed in this very straight forward equation here,
00:50:14.20 which is saying that the standard free energy that can be gained
00:50:18.04 from a process where there is electron transfer between the electron donor and the electron acceptor
00:50:23.13 is a function of the number of electrons transferred,
00:50:26.22 multiplied by the Faraday constant, and this in turn multiplied by
00:50:32.21 the difference in redox potential between the electron donor and the electron acceptor.
00:50:37.21 So for example, a common intracellular reductant, is NADH,
00:50:43.09 and in its oxidized state this is NAD+.
00:50:45.10 The redox potential of this redox pair is very low.
00:50:51.07 It is very negative on the electron potential scale that is typically expressed in millivolts.
00:50:57.07 On the other side of this scale are the electron acceptors with very high redox potentials, like oxygen.
00:51:04.20 And so when you couple through the membrane
00:51:09.27 a process of electron transfer from NADH to oxygen, thermodynamically you have the potential to generate
00:51:16.16 a lot of energy, and this is captured through a beautiful sequence of proton carriers and electron
00:51:24.14 transfer biomolecules contained within these membranes.
00:51:28.28 But these electron transport chains need not be between NADH and oxygen,
00:51:33.26 you can have a whole assemblage of things that can interrelate and so the minimum amount of energy
00:51:39.13 that needs to be supplied has been calculated. And this is a very crude estimation,
00:51:45.03 but it is an interesting study, and I refer you to below where you can see the reference.
00:51:49.05 Where for organisms operating in very low energy regime,
00:51:55.03 it was inferred that the minimum free energy required to sustain them and their growth
00:52:00.29 was about -4 kilojoules per mole, and that is about as low as you can go at least as experimentally measured.
00:52:06.21 Finally, regardless of the thermodynamic potential there are two other very important factors to keep in mind.
00:52:14.02 The second point is that the substrates themselves must be bioavailable. And so this is more of a kinetic problem
00:52:21.07 where we need to consider accessibility and transport of substrates
00:52:25.17 across the membrane to the site in the cell where they are used.
00:52:28.21 Or the ability of the cell to figure out a way to access them even if they can't transport them inside.
00:52:34.05 And the final point is that these substrates or the products after the metabolism has done its thing
00:52:42.00 must not themselves be toxic. So in the next couple of sections of this lecture,
00:52:47.26 I am going to give you examples of different microbial metabolisms to illustrate
00:52:52.12 these general points I have been making,
00:52:55.02 but I hope what you will remember from this seminar is the four big points about microbial diversity.
00:53:01.03 One, that it is incredibly ancient and over this long period of Earth history
00:53:06.08 numerous microorganisms in ubiquitous environments
00:53:09.18 have evolved diverse metabolisms that allow them
00:53:13.00 to catalyze fascinating chemical reactions and that these reactions
00:53:16.23 have affected not only the ability of the cells to grow and divide,
00:53:20.07 but in many instances have profoundly affected their environment,
00:53:24.09 be that environment one in an ancient ocean,
00:53:27.05 or today inside the human body.
00:53:30.18 Thank you.