Primordial Stone Soup
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 Ciénegas, 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 Ciénegas can help us learn more about the evolution of ancient life on Earth, and may even provide clues for finding life on other worlds.
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 Ciénegas 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 Ciénegas, looking at the ribosomal RNA, and for over 50 percent of them their closest relatives were marine organisms.
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 Ciénegas 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.
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 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.
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 Ciénegas 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.
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.