Even Dante would blanch at the conditions kilometers below the earth's surface. Temperatures climb past 100 degrees Celsius, pressures hundreds of times greater than atmospheric pressure bear down, and space is so tight even microorganisms can barely budge.
Yet, even there life persists. Now subsurface scientists have begun to identify the factors that determine why microorganisms survive deep underground in some places, but not others, report researchers from the Department of Energy's Idaho National Engineering and Environmental Laboratory and Princeton University. The INEEL specializes in subsurface science as part of its environmental mission.
High temperatures ensure nothing can live too far below the earth's surface. But pressure, the availability of water, the porosity of the surrounding rock and the flow of chemical nutrients also limit where extremophiles--microorganisms that relish harsh conditions--can exist.
As recently as the 1980s most microbiologists thought nothing could survive far below the soil layer. They now know extremophiles live embedded in rock hundreds of meters below dry land, in deep ocean sediments and in fissures crisscrossing the ocean floor.
"We're at the point of recognizing that microorganisms have remarkable abilities to colonize these environments and trying to understand the parameters that control that colonization," said INEEL microbiologist Rick Colwell, who presented a synthesis of recent findings in the Biogeoscience: Deep Biospheres: Where and How? poster session today at the American Geophysical Society meeting in San Francisco.
A better understanding of how extremophiles survive deep underground may shed light on how life endured the earth's violent youth, or show scientists where to look for life on other planets, said Princeton geochemist T.C. Onstott.
Temperature appears to be the primary factor in limiting how deep extremophiles can go. No known microorganism can live for long at 120 degrees Celsius. Since the surface temperature averages 15 degrees Celsius and the temperature in the ground increases with depth about 19 degrees Celsius every kilometer, extremophiles should die off between five and six kilometers below the surface of dry land.
Near geothermal hotspots, subsurface temperatures rise much more rapidly, and life survives only near the surface. In deep ocean sediments, where kilometers of cold water above keep temperatures down, extremophiles may thrive at depths of several kilometers below the ocean floor.
Pressure limits the range of extremophiles less than temperature does. Most microorganisms can survive pressure 600 times atmospheric pressure, which corresponds to a depth of six kilometers. At that depth in most locations the temperature likely exceeds 120 degrees Celsius.
Lack of water and chemical nutrients likely prohibits deep subsurface life in arid, geologically stable regions. For instance, little grows between the surface and the groundwater of the Snake River Plain, on which INEEL sits.
Conversely, extremophiles may be more abundant deep in active geological features, such as faults, mid-ocean ridges and salt deposits, where fluids and nutrients flow more freely.
Subsurface environments are so austere some extremophiles live in a state of nearly suspended animation. Microorganisms living on the surface divide after hours or days. Those living deep underground may divide only after hundreds of thousands of years.
Life may have persevered below the surface 4 billion years ago, when asteroids routinely pelted the earth and caused the oceans to boil, Onstott said, so microorganisms living deep underground may provide clues about the emergence of life on the developing planet. "If you want to understand primitive microbial ecosystems, the only place you can go is into the subsurface," he said.
If life exists elsewhere in the solar system, it may be tucked beneath the surface of other planets or their moons. By studying subsurface extremophiles on earth, researchers may learn where to look in their search for extraterrestrial life.
As data accumulate, researchers hope to understand the processes that control life deep below the surface well enough to predict what they will find in a given locale. "In the early days it was discovery-driven science--let's drill a hole and see what we find," Colwell said. "I think it's now moving into what I'd call hypothesis-driven science."
Onstott agrees. "In terms of microbiology," he said, "I think the field is headed toward identifying energy sources for these microorganisms, correlating these sources to microbial activity and determining whether that activity has changed the subsurface environment."
The INEEL is managed and operated by Bechtel BWXT, Idaho, LLC (BBWI) for the U.S. Department of Energy.
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