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. High temperatures ensure nothing can live too far below the earth's surface. 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. 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.