Big Clues from Small Slimy Creatures

Ferran Garcia-Pichel doesn’t just study microorganisms, he studies how they interact with and change their surroundings. He is searching for clues about the first type of organism to dominate the continents of the Earth.

To date, he’s found some intriguing clues. The descendants of those early organisms still stand in the deserts of the Southwest. Garcia-Pichel thinks that the modern organisms could be similar to the last creatures left standing in the case of a future drastic global change.

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

Garcia-Pichel is a geomicrobiologist at Arizona State University. He studies microscopic, single celled cyanobacteria—one of the oldest life forms on Earth. Cyanobacteria no longer dominate the planet, but they are well known from their fossils. And they still live among us, forming soil crusts in the Arizona desert.

The Earth is about 4 billion years old. Stromatolites are the fossilized remnant of large aquatic communities of cyanobacteria. Stromatolites are the most common fossils from the Precambrian period They’ve been around for almost 3.5 billion years. That’s more than 75 percent 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 planet’s evolution.

Geochemical evidence shows scientists that cyanobacteria also seem to have played a major role in transforming the early Earth into the planet we know today. Cyanobacteria invented oxygenic photosynthesis. They helped transform the biosphere.

By studying fossilized cyanobacteria, scientists are studying 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.

Garcia-Pichel says that cyanobacteria’s role in the transformation of early life on land has not been duly recognized. Land-dwelling 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. An organism might leave a variety of biosignatures 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.

Garcia-Pichel says that if scientists can learn how to recognize the signatures left behind by living terrestrial cyanobacteria on our planet today, it might then be possible to look for similar signatures in the Earth’s geologic record, as well as on other planets. The key to finding that evidence is in figuring out the signatures.

The ASU scientist has an idea. Because cyanobacteria lacked competitors, the early Earth’s surface may at one time have been completely covered with the organisms. But, as the diversity of life increased, things changed.

The advent of leafy plants caused problems. Fallen leaves blocked much needed sunlight from hitting cyanobacterial communities in the soil. In short, they were crowded out.

Garcia-Pichel and his colleagues work in Arizona for a reason. In modern Southwestern deserts, aridity limits plant development. Soils are usually bare. But cyanobacteria still thrive in communities known as soil crusts.

In some of the more pristine portions of the Arizona desert, cyanobacteria withstand desiccation by living one or two millimeters under the surface. From time to time, they get wet and come to the surface for a couple hours of activity. It is during these wet periods that they multiply through cellular division and repair all the damage from the dry period.

During dry spells, the cyanobacteria lay dormant. They 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.

Cyanobacteria are too small to see with the naked eye. But Arizonans definitely see their effects. Construction and agricultural activity disturbs the soil and breaks apart cyanobacterial communities. As a result, dust storms in the Phoenix area get bigger and nastier.

Garcia-Pichel and his colleagues plan to study modern desert soil crusts. The crusts are a common and important remnant of cyanobacterial ecosystem. Cyanobacteria may be old, but the science and methodologies the ASU scientists use for their geomicrobiological research are new. Researchers are just beginning to understand how microbes can and probably do drive biogeochemical cycles.

“Geomicrobiology is really a novelty in the Western world,” Garcia-Pichel says. “People still are amazed that microbes can do things with minerals and rocks.”

Novelty or not, Garcia-Pichel thinks that geomicrobiology will help scientists understand the evolution of the Earth. Such understanding might help them understand the evolution of other planets in this solar system and beyond.

“Think about Mars,” Garcia-Pichel adds. “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.”

The chances of finding some indirect evidence of life on Mars might be good if we know how to look closely at surface soils.

Garcia-Pichel adds, “We have to learn how to recognize possible biosignatures from these ecosystems.”—Matthew Shindell