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Where Biosphere Meets Geosphere

Microbes don't only affect other living things. They break down rock, construct mineral deposits and create by-products ranging from electrical currents to methane gas
aging



J. W. Stewart
In the few hundred years since Antonius van Leeuwenhoek first observed bacteria under the microscope, we humans have worried most about what these organisms do to us. Recently, though, scientists have begun to study how microbes affect our environment¿and as it turns out, these bugs do an awful lot. Having existed on the earth for the past 3.5 billion years or so, microbes have continuously helped to shape the planet: they break down rock, construct mineral deposits and create by-products ranging from electrical currents to methane gas. Moreover, they often do so in the most inhospitable surroundings.

rocks

EARTH'S TERRAIN is constantly being shaped by the actions of microbial communities. Scientists now know that bacteria break down rock, construct mineral deposits and create by-products ranging from electrical currents to methane gas.

"If they don¿t make us sick, then we¿ve typically ignored all these bacteria," says Jennifer Roberts Rogers of the University of Kansas, "but there¿s a huge biomass of bacteria living beneath our feet in the rock." Researchers now estimate the total number of prokaryotes (bacteria and Archaea) living in the earth¿s subsurface to depths of 4,000 meters at about 3.8 X 1030 cells. (In comparison, an average human body contains approximately 1015 cells.) "If you want to go out on a limb, you could make a case for bacteria being everywhere," Rogers says. "So you could make the case that bacteria are involved everywhere, whether directly or indirectly," in the geologic cycle.

When it comes to geologically active microbial communities, among the most interesting are those that live without oxygen and don't rely on photosynthesis. The first well-studied examples reside in such harsh environments as Yellowstone¿s hot springs and "black smokers" along midocean ridges, formations that spew 80-degree-Celsius water from sulfur mounds. These so-called extremophiles, which were made famous in the 1990s, depend on high temperatures and sulfur to survive. Their discovery first suggested new ways in which life may have started on the earth.

Sulfur Caves

Libby Stern, a geochemist at the University of Texas at Austin, and co-worker Phillip Bennett, now study a sulfidic environment in a cave near Lovell, Wyo., that Stern says is analogous to the hydrothermal vent ecosystems on midocean ridges. "This cave has no sunlight; the ecosystem is chemoautotrophic," she says. "There¿s no photosynthesis, only chemistry. Life is derived from the rocks and the chemistry of the groundwater."

A diverse collection of microbes thrive in the dark cave. Annette Summers Engel, a graduate student whom Stern advises, says that she has categorized six or seven different groups of microbes, according to their metabolic activities. A hydrogen sulfur-rich stream runs through the center of the cave. "Where the water comes in, it¿s anaerobic, so the microorganisms are anaerobes," she says. "In a few meters, only aerobic microbes grow in the surface of the mat. We¿re talking about a 10- or 15-meter stretch, and the microbial mats are even more complex." Within the space of a teaspoon, oxygen amounts vary vertically as well as horizontally. Within two millimeters, Engel has observed different layers of microbial communities.

Stern says the carbon ratios of the cave¿s microbes show very different values from thsoe associated with photosynthesis¿a variation she attributes to the different means of CO2 fixation these chemoautotrophic bacteria use. "The resulting organic matter in their tissues," she adds, "may be a biomarker for this process." This biomarker might allow geologists to determine what kinds of life were present at the time a rock was deposited and could also give a snapshot of the environmental conditions then.

The Dolomite Mystery

rocks

DOLOMITE is missing in many places where you would expect to find the mineral, including marine waters. As scientists learned in 1995, though, sulfate-reducing bacteria are required for the creation of dolomite in oxygen-lacking, salt-rich environments.

Microbiology has apparently solved at least one geological mystery to date: that of dolomite¿a soft, white, vitreous mineral composed of calcium, magnesium and carbonate crystals. By all accounts, the world should be covered with the stuff. Huge dolomite deposits do form sedimentary rock in the American Midwest, as well as in parts of Europe and Mexico. But in many places where you would expect to find the mineral¿marine waters, for example, usually contain an overabundance of the necessary ingredients¿it is entirely missing.

"Dolomite shows up in the rock record all over the place, but it rarely occurs in modern low-temperature environments," Rogers says. "There are some very special environments, hypersaline environments that do have dolomite precipitation," such as Rio de Janeiro. But it¿s hard to find a place where dolomite is forming now, never mind at rates that explain large deposits in the past.

In 1995, Crisogono Vasconcelos of the Swiss ETH-Central Geological Institute and co-workers seemed to solve the puzzle of the missing dolomite deposits, publishing a landmark paper in Nature that documented just how the mineral forms in a laboratory setting. The magic ingredient? Microbes. Subsequent papers by the same group pinpointed a connection between sulfate-reducing bacteria and the creation of dolomite in an oxygen-lacking, salt-rich environment.

Judith McKenzie, one of Vasconcelos¿s colleagues, proposes that dolomite should be considered a biomineral and thus not entirely inorganic. "It¿s a clue as to how dolomite may have been precipitating in geologic time millions of years ago," Rogers adds. "If it always was precipitating this way, maybe there were more bacteria around to do the work or, if things have changed enough, that dolomite precipitation actually happens differently at low temperature in the modern [era] than it did through geologic time."

From Bugs on Earth to Life in Space

There remains much to be learned about geologically active microbes. In Dianne Newman¿s laboratory at the California Institute of Technology, scientists are working to determine how microbes can subsist on inorganic minerals by, for example, gathering electrons from the iron in goethite. Currently, Newman says, "very, very little is known about the enzymes that they use to do this, much less what genes encode those enzymes." Microbial activity may also affect our atmosphere and seas, she says. "You can go from the bottom of the ocean floor all the way to the outer atmosphere and try to figure out whether trace gases that affect global warming are coming from microbiology."

At this past December's meeting of the American Geophysical Union (AGU), oceanographer John Delaney of the University of Washington at Seattle, a major contributor to hydrothermal vent ecology research, spoke of "conceptual shifts" regarding microbial biospheres. "In 11 of 11 cases, underwater volcanoes have been found to have a massive effusion of biomaterial," he noted, and the enzymes that allow these extremophiles to survive in such conditions "could have a profound impact on pharmaceuticals and industrial processes." Delaney, who now specializes in astrobiology, suggests that plate tectonics modulate the marine microbial biosphere in ways we don¿t yet understand and our oceans may hold examples of what life looks like elsewhere in the universe.

Rogers says that "biological imprints" in the rock record would allow geologists to determine available nutrients, the global carbon cycle and diagenesis (how quickly new rock is being made from old rock) not only on this planet but on other planets. "We might be able to find traces of life using basalt as a proxy," she says, because microbes may be intrinsic to its weathering cycle. Back on the earth, her research focuses on bacteria in soils or that can be used in bioremediation. She has found that some bugs may break down the organic compounds of an oil spill more quickly in the presence of inorganic minerals. "When you get to low temperatures or environmental temperatures, not deep-sea vent temperatures, but regular settings," she says, "I think bacteria could be a dominant mechanism for the weathering and breakdown of rocks because they are able to overcome kinetic barriers."

These tiny bugs may "impact the entire planet," according to a report released this month by the American Society for Microbiology on "the interface between the biosphere and the geosphere." The field remains wide open and could lead to practical solutions for environmental disasters or to answers about the origins of life. As Delaney says, "Not much is known about what¿s left to be discovered."

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