A critical chemical reaction previously thought to support microbial life deep below Earth's surface, and possibly on Mars, is in fact highly unlikely. The findings are reported in last week's issue of the journal Science by researchers funded by the National Science Foundation (NSF)'s Life in Extreme Environments (LeXeN) program and affiliated with the University of Massachusetts at Amherst (U. Mass.).

"This is an important step forward in our continuing efforts to understand the processes that sustain life deep beneath the earth's surface," says Mike Purdy, director of NSF's LeXeN program. "Negative findings like this are as important as positive ones in their importance to our understanding of the processes that determine the limits to life."

It had been generally accepted by scientists that hydrogen gas produced from rock could provide energy to support the growth of microorganisms living below Earth's surface, says U. Mass. microbiologist Derek Lovley. The hydrogen was thought to be produced when basalt, a common form of rock, reacts with water.

However, a research team led by Lovley has found that this concept is incorrect. Although hydrogen gas can be produced from basalt under artificial laboratory conditions, there is no hydrogen production under the conditions actually found in Earth's subsurface.

Lovley and his colleagues found that hydrogen could only be produced from the basalt when the rock was exposed to acidic conditions -- but environments containing basalt are never acidic.

"The idea that hydrogen produced from rocks could support large subsurface microbial ecosystems on Earth and possibly other planets was fascinating and was accepted by most microbiologists," Lovley says. "Unfortunately, this concept can not be supported by the available data."

From analyses of chemical and microbiological data, Lovley and collaborators Robert Anderson, U. Mass. graduate student, and Francis Chapelle, a hydrologist at the U.S. Geological Survey in South Carolina, suggest that the microorganisms are probably living on organic matter associated with the rock, not hydrogen. This is similar to the way that microorganisms grow in soil on Earth's surface.

The scientists emphasized that even though the microorganisms living deep in the Earth may make a living in a manner similar to that of surface microorganisms, they may have other unique characteristics. For example, Lovley's recent research has demonstrated that microorganisms from the earth's subsurface can be used to remove radioactive metals, as well as hydrocarbons from polluted groundwater.

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