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.

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.