Primordial Stone Soup
8 Jan 2007
(Source: Astrobiology Magazine)
Astrobiologists who try to track fossils back in time, hoping to find clues to the origin of life, often must end their pursuit at stromatolites. These fossilized structures are the earliest evidence for life on Earth.
The oldest stromatolites date back 3.5 billion years. Although there is some controversy about whether these more ancient structures were produced by microbes or by non-biological processes, various lines of evidence suggest they were indeed formed by life.
To create a stromatolite, microbes such as cyanobacteria form mat-like communities in water or in tidal zones, and when too much dirt and other material becomes trapped in the mats and limits the amount of sunlight that can filter through, the bacteria migrate up and form a new community on top of the old. Over time, the multiple layers stack up to form the large stone-like structures.
There are still stromatolites being constructed by microbes today, although usually only in biologically isolated regions where the bacteria don???t have to compete with other life forms.
Janet Siefert, a biologist and statistician at Rice University in Houston, Texas, has been investigating stromatolites in Cuatro Ci??negas, a valley in the heart of Mexico???s Chihuahua desert. The valley???s isolation has made it biologically unique, with life evolving separately in the many spring-fed pools, lagoons and marshes over millions of years.
In this interview with Astrobiology Magazine???s Leslie Mullen, Siefert describes how the modern stromatolites of Cuatro Ci??negas can help us learn more about the evolution of ancient life on Earth, and may even provide clues for finding life on other worlds.
Astrobiology Magazine (AM): In Cuatro Ci??negas, you???ve been studying stromatolites that are forming in and around the remains of an ancient ocean.
Janet Siefert (JS): There are various pools in the valley, and around the edges of these pools are different types of stromatolites. According to the geologists, there???s a big body of water underneath the valley, and the overlying substance acts like a sponge, soaking up that underlying ocean and causing the pools to form.
Even though the stromatolites seem to be made up of similar bacterial communities, they have different morphologies. Rimmed around one pool are shelf stromatolites, and then in the water are mounded ones that look like domes. Those go down to about 40 feet. There are also little round ones that look to me like tumbleweeds, and they are in an above-ground river system. None of them have that layered look that you typically see in stromatolites.
AM: Are the different formations due to the environment, or due to the different microbes that make the stromatolites?
JS: I wish we knew. Cuatro Ci??negas is like a Galapagos. There???s not a lot coming in and going out of that mountain range, and because it???s constrained by the geology, it???s like a living laboratory. It has 70 macrofauna that are endemic to the region, and we???re interested in finding out if there are endemic microfauna as well.
The area was part of the proto-Gulf 100 million to 65 million years ago. As the Gulf of Mexico began to form, the mountain ranges pushed up, entrapping the seawater into this region that???s now 700 meters above sea level. Valeria Souza published an article in PNAS substantiating that the ancient water is still there. They analyzed the microfauna in some of the places at Cuatro Ci??negas, looking at the ribosomal RNA, and for over 50 percent of them their closest relatives were marine organisms.
We also did metagenome studies. In one of the pools, the viral community is more closely related to marine viral communities than it is to anything terrestrial.
AM: But you said you???re not finding the layers that we???re used to seeing in the ancient stromatolites, or in the Shark???s Bay marine stromatolites in Australia?
JS: No. Stromatolites literally means layered, from ???stroma,??? and there is some very fine layering in some of Cuatro Ci??negas stromatolites. But if you should happen to tear off a piece, it would look more like a sponge. I think that???s because there isn???t a seasonal input of sediment that traps and creates layers. When I saw these stromatolites, I thought that if they became fossilized they wouldn???t be as spectacular-looking as the fossils in Australia.
AM: And you???ve been studying what causes stromatolites to turn into stone ??? to lithify.
JS: If we get a handle on what are the physical parameters that cause today???s microbial mats to lithify, then we can start to get an idea of how it may have happened in the past. That???s the astrobiological context -- understanding how these modern stromatolites are lithifying is relevant to early Earth. There probably weren???t cyanobacteria producing oxygen 3.5 billion years ago, so what kind of energetic constraints were there? What was the atmosphere like? What was the water chemistry that allowed for the production of those robust stromatolites?
Stromatolites have been lithifying for 3.5 billion years, so it must have developed pretty quickly. To find out how that happened, we can only look at what???s here now, which is mostly aerobic.
You can find stromatolites in hot spots like in Yellowstone; you can find them in Shark???s Bay where there???s an extreme salt environment. It???s more unusual to have stromatolites in colder freshwater environments. The water at Cuatro Ci??negas is bath temperature warm in a lot of the pools, so it???s being heated from way down deep, but Cuatro Ci??negas is still not hot like Yellowstone. It???s certainly not salty like Shark???s Bay. Green Lake in New York, a cold fresh water lake, has stromatolites. The only example I???ve found for stromatolites in an anaerobic setting was in a gold mining operation in New Zealand. So we only have a few places today where you have the same ancient processes occurring, allowing us to get closer to what the early Earth might have been like.
AM: We think the early Earth had only anaerobic bacteria, and also the environment of the Earth itself was so different from today. Do you really feel that you can use modern examples to answer our questions about the early Earth?
JS: We???ll never be able to constrain it completely by looking at modern examples. But we can extrapolate back to an anaerobic system if we can figure out the basic energy dynamics that are needed, how much biomass production there needs to be, over what period of time, and what kind of phosphorus supply does there need to be ??? figuring out what are the basic things needed to produce stromatolites that can lithify. How complex does the community have to be to produce lithification? Maybe it doesn???t require much complexity.
But we???re finding that for modern stromatolites, lithification does require a great deal of complexity. Lithification today occurs because of metabolism, where cyanobacteria produce oxygen and change the pH around the microbial mat so that instead of having the calcium carbonate in the water, it precipitates out into the mat. The other way to get lithification is for the mat to produce what we call an ???exopolymeric substance??? that traps sediment. Either of those two processes can make a stromatolite turn into stone. There can also be some mix of the two processes.
We???re trying to understand which processes are causing lithification at Cuatro Ci??negas. That also may help explain some of the morphological differences we???re finding there. So, for instance, there are stromatolites there that are lithifying now, and they???re doing so because the water chemistry is changing. That particular riverine water system is either starting to dry up, or it???s changing its course and so it???s moved and it???s creating a place where there???s more evaporative opportunity. Being able to catalogue what???s happening there ??? determining the changes in the water chemistry and in the cyanobacterial constituency ??? will help us better understand how lithification occurs.
We also have several ways that we could manipulate them. We put the little stromatolite balls in buckets and dump phosphorus in, or do whatever we want to change their ecology, and compare their dynamics to the stromatolite communities within the valley.
When I first started out, I thought lithification was not going to take too much sophistication. But it turns out it???s a complex system, because it happens either through metabolism or from the creation of this exopolymeric substance.
AM: Is the exopolymeric substance plastic-like, similar to kerogen?
JS: No. It???s not mucus, but it???s sticky. It???s amazing, because it makes a connection between organisms. This gooey stuff facilitates lots of things, like communication signals and horizontal gene transfer. It creates an ???outside the cell??? communication contact system.
By getting information about how these modern oxygen photosynthetic mats are working, maybe we can extrapolate back to the earlier forms. Frances Westall says that the exopolymeric substances are preserved in stromatolites that are 3 billion plus years old.
AM: If lithification does require a certain level of complexity, does that mean we???re extremely limited in using fossils to track back to the origin of life?
JS: Going back to the origin of life is difficult because it is so different from existing life. It???s hard for us to conceptualize, in our aerobic atmosphere and environment, what the ancient Earth might have been like. Genetically, my work used to center on looking at gene sequences, and trying to extrapolate back to early microbial metabolisms. But you???ll never be able to assume a modern metabolism was also in a very early organism.
AM: I???ve heard of some studies that tried to isolate organisms out of ancient stromatolites, but they haven???t had much luck.
JS: We even have trouble isolating organisms out of modern stromatolites, because it???s a community. These things have been living together for a long time, and they don???t want to be separated from their partners. We would???ve already had some of the cyanobacteria isolated from the Cuatro Ci??negas samples, but it???s really hard to get pure cultures from stromatolites.
AM: It???s amazing that there may have been such complex microbial communities dating back to 3.5 billion years ago, and yet we think life only arose 3.8 billion years ago.
JS: Isn???t that striking? When I look at the molecular data and try to go back in time, the bottom line is that it all seems complex really quickly. Oxygenic photosynthesis is pretty complex, but even if you don???t have that, methanogenesis is complex! There doesn???t seem to be any place where it went through sequential steps from something really primitive.
I???ve worked with Jim Kasting to model how you get enough greenhouse gases to counteract the faint young sun, and I always tease him and say it???s a ???Methanogen World.??? But even that doesn???t make sense to me, because there aren???t many places on Earth where you have a pure culture of just one organism. It???s not the way it works. There???s always an intense interactive community.
Abigal Allwood recently discovered a 10-kilometer section of a nearly 3.5 billion-year-old microbial reef system in Australia. That???s not a separate little oasis -- life was a community even back then. So maybe that gives credence to thinking about the origin of life as being a community of cells. There???re some problems with that, but the complexity of life, for as far back as we can look, is sophisticated quickly.
In addition to understanding early life on Earth, maybe our studies could provide some clues for finding life on another planet. Anywhere there???s a planet in a habitable zone, it???ll be working with the same elemental toolbox that we have here on Earth. Phosphorus is a great energy carrier, carbon does a lot of good things, and so on. Lithification just takes advantage of chemistry -- you get the right things in the water, and you can lithify. But is there enough calcium and carbonates or silica around for that to happen? If a planet does have the necessary ingredients, you probably would be able to find similar processes occurring.