Common desert organisms may help in search for life in space


ASU geomicrobiologist Ferran Garcia-Pichel doesn't just study microorganisms, he studies how they interact with and change their inanimate surroundings. In his work he searches for clues about the first type of organism to dominate the early earth continents -- clues he says lead him to believe their descendants, still standing in the deserts of the Southwest, could be similar to the last ones standing in the case of a drastic change.

Well, maybe not standing. These tough little critters, which thrive in desolate places and extreme conditions, have no legs.

They are the microscopic, single celled cyanobacteria -- one of the oldest life forms on Earth. While they may no longer dominate the planet, they are well known from their marine fossil stromatolites, and they still live with us here, forming soil crusts in the Arizona desert.

Along these lines, Garcia-Pichel and his collaborators will be presenting several papers and posters at the 2003 NASA Astrobiology Institute General Meeting at Arizona State University. These are on the topics of cyanobacterial community structure, microbial mats, desiccation under different conditions, molecular analysis of calcifying cyanobacteria, comparisons of rDNA sequences from cyanobacteria, as well as the analysis of microbial nitrogen cycling in desert soil crusts.

The Earth is about 4 billion years old. Fossilized stromatolites (large deposits left by aquatic communities of cyanobacteria), the most common fossils from the Precambrian period, are known for about 3.5 billion years. That's more than 75% of the earth's history. The size and number of these fossils indicate that cyanobacteria once formed the major ecosystems of the Earth. They were witness to nearly every stage in the earth's evolution.

Geochemical evidence shows that cyanobacteria also seem to have played a major role in transforming the early earth into the earth we know today. They invented oxygenic photosynthesis and through this turned the earth's biosphere from reducing to oxidizing.

Studying fossilized cyanobacteria allows us to study what life was like and how it evolved on what was in many ways a different planet. But there may be more to the story. According to Garcia-Pichel, their role in the transformation of early life on land has not been duly recognized, perhaps because terrestrial cyanobacteria do not fossilize as readily as their marine counterparts.

Scientists might have to look at indirect evidence -- a unique sort of chemical or mineral signature (known as a biosignature) that an organism leaves in its environment. The study of living cyanobacteria and the chemical biosignatures they leave in their environment is essential for the task of looking for past-life evidence on other planets and our own.

According to Garcia-Pichel, if we can learn how to recognize the signatures left behind by living terrestrial cyanobacteria on our planet today, we will soon be able to look for them in our geologic record, as well as on other planets. The key to finding their evidence is in figuring out their signatures.

Garcia-Pichel hypothesizes that, because they lacked competitors, the early earth's surface may at one time have been completely covered with cyanobacteria. But, as the diversity of life increased, things changed. The advent of plants with leaf-litter blocked much needed sunlight from hitting cyanobacterial communities in the soil. In short, they were crowded out.

And this brings Garcia-Pichel and his colleagues to the deserts of the Southwest, where aridity limits plant development, soils are usually bare and cyanobacteria still thrive in communities known as soil crusts.

In some of the more pristine portions of the Arizona desert, cyanobacteria essentially lead an existence of withstanding desiccation and the insults of the environment, living one or two millimeters under the surface. From time to time, every monsoon or so, they get wet and come to the surface for a couple hours of activity. In these wet periods they multiply through cellular division and repair all the damage from the dry period.

During dry spells, the cyanobacteria (as well as other microbes that live within their community) just lay dormant and suffer what Garcia-Pichel calls "the slings and arrows of the Southwest." To minimize the effects of these slings and arrows, they secrete slime. This slime creates a crust that holds the soil of the cyanobacterial ecosystem in place. This slime essentially cements the desert crust and stabilizes the soil, keeping it from blowing away in the wind.

Though cyanobacteria are too small to see with the naked eye, we definitely see their effects. Dust storms in the urban Arizona, as well as the unacceptably high particulate matter count, are enhanced by the loss of cyanobacterial soil communities from agriculture and construction.

Garcia-Pichel and his colleagues plan to study modern desert soil crusts, a common and important remnant of cyanobacterial ecosystem, and how they undergo diagenesis -- the chemical transformations that occur after burial.

Cyanobacteria may be old, but the science and methodologies Garcia-Pichel and his colleagues incorporate in geomicrobiological research are new. According to Garcia-Pichel we are just beginning to understand how microbes can and probably do drive biogeochemical cycles. "This area of geomicrobiology is really, in the Western world, a novelty," says Garcia-Pichel, "people still are amazed that microbes can do things with minerals and rocks."

Novelty or not, Garcia-Pichel and his colleagues believe geomicrobiology will help scientists understand the evolution of the earth, and the possible evolution of other planets in this solar system and beyond.

Speaking of the applications of their research to the question of Mars, Garcia-Pichel said, "If we postulate that water was not always relegated and then became relegated, and that microbial life was present, what kind of ecosystem would have been the last to be on the surface soils of Mars? It would have been something like desert crusts today. So just there our chances of finding some indirect evidence of life are highest, if we just know how to track, how to teat and how to recognize possible biosignatures from these ecosystems."

The NASA Astrobiology Institute is composed of over 700 researchers distributed at more that 130 research institutions across the United States. Its central offices are located at NASA Ames Research Center, in the heart of Silicon Valley, California. Additional information about the NAI can be found at its website: http://nai.arc.nasa.gov

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