Living in the Dark
A photo of the test site in the Beaverhead Mountains.
Credit: Derek Lovley
Over the past several years, scientists have discovered life in the most unusual places. From rocky abodes deep underground, to hot volcanic vents under the seas, there seems to be no place on Earth that life doesn’t exist.
All of this life, even the life that lives in total darkness, is dependent on the Sun for energy. Plants and many Bacteria get their energy directly from sunlight, through photosynthesis. Animals and other organisms get their energy indirectly, by feeding on the complex organic molecules of photosynthetic organisms. These sun-produced organics eventually filter down into the Earth’s darkest reaches, providing food for even the deepest living organisms.
Now, scientists have discovered an ecosystem where sunlight or organic molecules are not needed for energy. Deep underneath the Beaverhead Mountains in Idaho, one-celled bacteria and archaea live in the 58.5-degree C (137 F) waters of the Lidy Hot Spring. The microbes living in these waters are completely cut off from organic carbon and sunlight. Instead, the microbes exist by combining hydrogen and carbon dioxide gasses dissolved in water. This chemical reaction produces methane and generates the energy needed to sustain life.
A research team led by Francis Chapelle of the U.S. Geological Survey and Derek Lovley of the University of Massachusetts found this unusual community of methanogenic organisms. The team reported their findings in the January 17 issue of the journal Nature.
"The microbial community we found in Idaho is unlike any previously described on Earth," said Lovley. "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."
Archaea are considered to be the organisms most closely related to ancient life on Earth. Microbes similar to Archaea were probably prevalent on the Earth early in its history, when organics were scarce and hydrogen was more abundant.
Life could exist in pockets of underground water on Mars, or in an ocean under the frozen crust of Europa.
Hydrogen-eating archaea exist today in extreme environments, such as deep-sea geothermal vents, but the microbes are always outnumbered by organisms that rely on sunlight for their energy. Scientists weren’t even sure if an ecosystem dominated by methanogenic archaea would be possible.
In the Idaho geothermal hot spring, however, archaea make up over 90% of the population.
According to Lovley, scientists have searched for at least a decade for a community of microorganisms that could survive on hydrogen rather than sunlight or organics. Such a community could teach us about potential life on Mars or on Jupiter’s moon Europa.
Life could exist in pockets of underground water on Mars, or in an ocean under the frozen crust of Europa. Since this life would exist below the surface, it wouldn’t receive any sunlight and therefore would need an alternative means of fuel.
"In prior studies," said Lovely, "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. Those environments are not likely to represent conditions on Mars because, on Mars, such organic matter would not be available."
Chapelle said his team searched for hydrogen-based ecosystems at about a dozen other sites, including hot springs in Virginia and at Yellowstone National Park. The volcanically active Yellowstone region is home to 80 percent of the world’s geysers and half of its geothermal features.
The team finally arrived at the Lidy Hot Springs in Idaho. With landowner Charles Wilson’s help, they drilled a hole 200 meters (656 feet) into the rock to where the hot springs originate. They brought up samples of the microorganisms and then took them back to the lab for DNA sequencing.
Lovley and his research team plan to concentrate only on the Lidy Hot Spring and are now beginning to study the physiology of the methanogens that they collected.
Credit: Stan Sherer
The concentration of microorganisms was small–less than the amount of Bacteria in ordinary tap water–but the results confirmed that a community of Archaea could survive without sunlight or organics.
"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," 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, based on what they know of Martian subsurface chemistry."
The Mars Odyssey spacecraft has found some suggestions of extensive hydrogen deposits within 1 meter (3 feet) of the surface of Mars. Other studies have shown that Mars and Europa both might have hydrogen-rich environments.
"Life requires water and an energy source," said Lovely. "On Mars and other planets or moons in our solar system on which life might exist, liquid water is only available below the surface where there is no sunlight. So, if there is life, it must sustain itself with alternative energy sources. Now that such a community has been discovered on Earth, 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."
Lovely says that although it is not yet known how the bacteria in the Lidy Hot Spring feed themselves, the bacteria constitute a very minor portion of the microbial community.
While other places on Earth may harbor similar environments, and therefore similar methanogenic archaea, the research team plans to concentrate only on the Lidy Hot Spring. Lovely says they are now beginning to study the physiology of the methanogens that they collected.