In a culmination of a decade-long quest, investigators have discovered a unique colony of ancient microbes thriving in geothermal hot springs bubbling 200 meters below ground in Idaho -- a geological environment reminiscent of the subsurface landscape of Mars.
The find may portend the presence of life elsewhere in the solar system or even beyond, the researchers said in the British journal Nature.
Shunning the sun, the tiny lifeforms instead generate energy from hydrogen in the rocks, producing methane as a by-product. While scientists have long been familiar with other such "methanogens" -- which inhabit everything from human guts to ocean-bottom sediments -- they have never before encountered an environment dominated by their ilk.
"The microbial community we found in Idaho is unlike any previously described on Earth," said study co-author Derek Lovley, head of the microbiology department at the University of Massachusetts in Amherst, Mass. "This is as close as we have come to finding life on Earth under geological conditions most like those expected below the surface of Mars."
The team identified the microorganisms -- members of a group of prehistoric relatives of bacteria called Archaea -- from their genetic makeup.
"We have discovered a microbial community living in the deep subsurface of a volcanic terrain which is dominated by microorganisms living exclusively by eating molecular hydrogen and by breathing carbon dioxide. This kind of metabolism has been hypothesized to have been the first to have evolved on the early Earth, and has been hypothesized to occur presently on Mars or (the Jovian moon) Europa," said lead author Frank Chappelle of the U.S. Geological Survey in Columbia, S.C.
These hypotheses can now be tested by actual observations, he added.
"The significance of our work is that we have shown that such microbial communities, which exist completely independently of solar energy -- unlike the hot springs on the ocean floor which are based on oxygen metabolism -- are indeed possible," Chappelle told UPI. "Furthermore, this microbial community may be representative of the kinds of life that initially evolved on the early earth, and which may presently occur on Mars or Europa."
Significantly, the findings imply hydrogen may be an important requirement for extraterrestrial life, investigators said.
"If hydrogen is indeed present on Mars in association with liquid water, the kind of metabolism we describe from (Idaho) may occur on Mars," Chapelle said.
Water and an energy source are the two key ingredients of life. On Earth, the primary energy source is sunlight, which plants convert into organic matter other organisms use for fuel. On Mars and other heavenly bodies on which life might exist, liquid water exists below the surface where there is no sunlight. So, if there is life, it must sustain itself with alternative energy sources.
"This study demonstrates, for the first time, that certain microorganisms can thrive in the absence of sunlight by using hydrogen gas released from deep in the Earth's surface as their energy source," Lovley said.
"They would be at the head of a food chain that might not be dependent on light, as opposed to being at the tail end of one that is," Jared Leadbetter, assistant professor of environmental microbiology, environmental science and engineering at the California Institute of Technology in Pasadena, Calif., told UPI. "Similar, 'dark,' hydrogen niches might abound elsewhere, near and far."
Geological formations where significant amounts of hydrogen are produced from rocks by chemical reactions usually occur in areas of active volcanic systems, said Gene McDonald of the Jet Propulsion Laboratory in Pasadena, Calif.
"Just how limiting this requirement is for Mars is not clear because we don't know if Mars is still volcanically active today. It probably has been active to some extent as recently as 200 million years ago or so because some of the Martian meteorites that have been recovered on Earth were formed around that time," he told UPI. "As for Europa, again we don't know if the satellite is volcanically active. There may be some activity due to the tidal forces acting on Europa, but this is not yet known for sure."
The demographics of the microbial community at Lidy Hot Springs below the Beverhead Mountains are like no others previously seen, for here, it is the hydrogen-consuming, methane-producing Archaea -- and not the bacteria -- that comprise the bulk of the population.
"At the Idaho site we saw something completely different," Lovley said. "Over 90 percent of the microorganisms were Archaea, which are microorganisms considered to be most closely related to ancient life on Earth. In this case, the Archaea were methane-producing microorganisms that live by combining hydrogen with carbon dioxide to make methane gas. They do not require organic carbon in order to grow. This is exactly the scenario that geochemists have predicted for life on Mars."
McDonald expressed surprise at the colony's apparent homogeneity.
"In many cases microbial communities are composed of several types of organisms that use each other's waste products for food, and methane oxidizing microorganisms are known to exist," he said.
The findings have been many years in coming.
"This is something people have been looking for for a long time, for at least the last decade or more," Tori Hoehler, research scientist at the National Aeronautics and Space Administration's Ames Research Center in Mountain View, Calif., said in an interview. "Lots of people believe there is a large and thriving biosphere below Earth's surface."
But such a spot would typically lie in the virtually inaccessible recesses, beneath hundreds of meters of rock, insulated from potentially harmful surface conditions, such as those on early Earth or current Mars.
"To understand how things might have been in our own early history or how they are on Mars, lots of folks have been looking for what's under the surface" without much success, Hoehler said. "Here, nature does the hard work for them, circulating the fluid through the hard rock and bringing it just below the surface."
Chappelle and company specifically selected the Idaho site for the similarity between its geological conditions and those expected on Mars.
"The water deep within these volcanic rocks has been isolated from the surface for thousands of years. It is devoid of measurable organic matter, but contains significant amounts of hydrogen," Chappelle said.
"In prior studies, when we looked in underground areas we considered promising, the DNA signatures of the bacteria present indicated they were living on organic matter carried in the groundwater or that had been deposited along with the subsurface of rocks," Lovley said.
"The microbial community found at the Idaho site is remarkably similar to what geochemists have postulated might be found below the surface of Mars," he added. "Now that such a community has been discovered, we can use it to test hypotheses about hydrogen-based subsurface life, and use these findings to develop strategies for searching for similar microbial communities on other planets."
