Session 2: Microbial Diversity and Evolution
Transcript of Part 1: Microbial Diversity and Evolution
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.