|Example of a layered stromatolite from the Ozark Precambrian. Most often, stromatolites are appear as variously-sized arches, spheres, or domes.
Credit: Ozarks Paleontology
Guerrero Negro, a small town of 10,000 located halfway down Mexico's Baja peninsula, is a popular destination for ecotourists. They come to gaze at the gray whales, or to marvel at the diverse population of shorebirds.
But in June, Dr. David Des Marais and his colleagues headed to the area to investigate an ecosystem not likely to be mentioned in any travel guide. Des Marais is a senior research scientist at the NASA Ames Research Center in Mountain View, CA, and a member of the NASA Astrobiology Institute. His research team made the trek south to study microbial mats, colonies of microscopic organisms 1- to 10-cm thick, inhabitants of a series of salt evaporation ponds that run along the Pacific shoreline near Guerrero Negro.
The Guerrero Negro investigation is a long-term project to study this primitive ecosystem. Des Marais and his colleagues believe the mats may hold important clues to what life was like on early Earth. They also hope to gain insight into how to search for signs of life on planets around distant stars.
Many scientists believe that for some 3 billion years after life first evolved on Earth, microbes were the only forms of life around. Only fragmentary evidence of that early life remains today, but geologists have found rocks that do provide such evidence. Known as stromatolites, these ancient fossils are notable because their layered appearance is reminiscent of the layering found in modern-day microbial mats.
"We focus on microbial mats," Des Marais explains, "because we have specific geologic evidence for their antiquity. We can go back to rocks that are almost 3.5 billion years of age and see films and features that are strongly consistent with microbial mat ecosystems."
But it's not that easy to find microbial mats these days. In most environments, microbes are eaten by other organisms or crowded out by plants before they can form stable mat communities. Only in certain extreme environments, too harsh for most of these grazers to live, can one find nearly pure microbial ecosystems. Guerrero Negro fits the bill because the water in the evaporation ponds there is so salty that microbial mats can compete successfully. The mats under study live in water 2 to 3 times as salty as seawater.
Because these modern mat communities are believed to function much like the ancient microbial communities from which stromatolites formed, says Des Marais, "they're good test beds for understanding early evolution."
But microbiologists can't just scoop up a piece of the mat, take it back to the lab and analyze it organism by organism. Most of the hundreds of different types of organisms that live in the Guerrero Negro environment have never been identified and may not ever be. To identify them, at least by traditional means, scientists must isolate and culture them. But because they require the environment of the mat to survive, they're difficult to culture. And it's often difficult to figure out the precise mix of conditions that each different organism requires.
|A hypersaline microbial mat from Baja California.
"For most organisms," say Des Marais, "being in a pure culture is an extraordinarily stressful situation. It's like putting you in a spacecraft and sending you to Mars with nobody around you. Extraordinarily stressful. You're a social organism. These guys are social, too. Their version of being social is a bit different from ours, though. It has a lot to do with sharing sunlight and exchanging chemicals between neighbors."
Moreover, he adds, "We're discovering that the very organisms that are the most important in this community are the hardest ones to grow in pure culture. Surprise, surprise: These are the best team players, therefore they have the hardest time living by themselves."
Life, fundamentally, is chemistry. All living creatures, from microbes to mammals, take in chemical nutrients and energy from the environment, reorganize it through a series of chemical reactions into useful forms and get rid of what they don't need. The basics of this process are well understood. The devil is in the details.
It is these details that Des Marais and his colleagues hope to tease out of the microbial mats in Guerrero Negro. They want to understand the inner workings of the biochemical cycles that are active in the mat community. By placing probes into the mat at different depths, they are investigating how various chemicals the most important are compounds containing carbon, oxygen and sulfur cycle through the system; how they are combined and recombined by the interactions of the mat organisms with each other and with their environment; how gases such as oxygen, methane and carbon dioxide build up and dissipate at different depths within the mat.
"How you actually control everything in response to environmental constraints is the essence of survival," says Des Marais. "And so the essence of ecological interaction is really the same thing: How are all these processes regulated as they interact with each other?"
|Microbial mat producing oxygen through photosynthesis.
Credit: UTA Department of Geology
Among the questions they are trying to answer is how the biochemical activity of the community changes over the course of a day. For example, the primary producers of organic material for the community are photosynthetic cyanobacteria. These organisms live at the top of the mat, where sunlight is available. They take in carbon dioxide from the atmosphere and the mat community and, using energy from sunlight, convert it to the organic carbon compounds that are the building blocks of living cells. In the process, they release oxygen. Thus, during the daylight hours, oxygen builds up in the upper layers of the mat.
But at night, this photosynthesis shuts down and other processes become dominant. The oxygen in the mat decreases as it is used by other organisms. These, in turn, release other gases, such as methane.
In an effort to quantify this process, Des Marais and his colleagues camped out near the salt ponds for several days and nights, carefully taking measurements every few hours, to learn how these interactions shift over the course of a 24-hour period.
One of the early results from the June expedition, Des Marais says, is that although the mats are submerged in oxygen-rich water, "a centimeter or two above the mat actually goes anoxic at nighttime. So oxygen is not available even to the surface of the mat at nighttime, which has to have some important implications for the organisms that can live there. For example, if you're an organism and you can't stand sulfide because sulfide is poisonous to you, you've got a problem living at the surface of the mat, because every night sulfide gets up into the water column."
Reserchers also measured the gases drawn from the atmosphere into the mat community, and emitted by the community back into the atmosphere. This information may shed light on a long-standing scientific debate: What was the composition of the atmosphere on early Earth and how did it change over time?
"There's the whole tapestry of early evolution that's wrapped up in understanding how these organisms interact and made this very efficient system work over a long period of time," Des Marais explains. "And then, of course, in so doing they leave markers of their remains in the sediments that you find them in, or they put gases into the atmosphere, which modifies the atmosphere."
Understanding the interaction of the mat community with the atmosphere may also help scientists who are planning a new generation of telescopes that will search for signs of life on distant worlds.
|Possible future telescope: TPF. TPF's spectroscopy will allow atmospheric chemists and biologists to use the relative amounts of gasses like carbon dioxide, water vapor, ozone and methane to find whether a planet someday could or even now does support life.
Within the next decade, space-borne telescopes should be capable of detecting Earth-like planets around other stars. But even with the powerful new telescopes, each planet will appear as nothing more than a tiny colored dot. To detect life on these as-yet-undiscovered worlds, the telescopes will make measurements that will indicate the presence or absence of certain atmospheric gases.
If they find a world whose atmospheric composition mirrors Earth's, it will be a strong indication that life, perhaps even complex life, exists there. But many scientists believe that for the first two billion years of life on Earth, our planet's atmosphere was very different than it is today. Studying the gases emitted by the Guerrero Negro microbial mats may help in determining what atmospheric signals from a remote planet might indicate a world more like that of early Earth than present-day Earth.
Other scientists on the Guerrero Negro expedition studied related questions. Jack Farmer, from Arizona State University (ASU), for example, looked at how sediments trapped by the mats form its characteristic layering. This may help in understanding the conditions that caused the layers seen in stromatolites.
Farmer also collected samples that he will examine for evidence of microscopic worms living within the mats. Some scientists speculate that these worms closely resemble the first animals to evolve on Earth, and that mat ecosystems were their first homes.
Another ASU scientist, Ferran Garcia-Pichel, took DNA samples from the cyanobacteria living in the mat. By sampling at different depths and over a wide surface area, he is hoping to learn which cyanobacteria are living where. This will aid future research at Guerrero Negro.
Garcia-Pichel is currently examining the DNA of the cyanobacteria to see how uniform their populations are from one location to another within the mat. If the populations of cyanobacteria are highly uniform within areas several meters square, Des Marais says, researchers at different labs will be able to examine different samples from the mat with a reasonable degree of certainty that "they are looking at the same ecosystem."
Pieter Visscher, from the University of Connecticut, and David Stahl and Matt Dillon, from the University of Washington, are performing similar studies of the bacteria that reduce sulfate and thus provide various sulfur species and gases to other members of the mat community.
Norman Pace and John Spear, from the University of Colorado, collected DNA from these mats to determine the variety, or "richness," of the microbial species present.
The work at Guerrero Negro will continue for many years to come. Genomics is one component that will be expanded in future visits to the region. This work will be headed by Mitch Sogin of the Woods Hole Oceanographic Institution.
Sogin hopes to discover which genes are being expressed at different locations in the mats at different times of day. By combining this data with that of Des Marais' and Pace's groups, investigators hope to create a more complete picture of how the organisms in the mat community interact.
Related Web Pages
Microbial Mat Research at NASA Ames Research Center (NASA Ames)
Microbial Mat Field Sites (NASA Ames)
Microbial Mat Page (UTA)