Microbes predicted for Mars found on Earth
Looking for a community of microbes that thrive on hydrogen and carbon dioxide is hard to do. The ideal conditions are thought to exist on Mars. And here on Earth most ecosystems are packed with organic carbon - energy bars for bacteria and humans, but not for the elusive microbes that might model life on other planets where organic carbon is in short supply.
Five years ago, Francis Chapelle and Paul Bradley of the U.S. Geological Survey (USGS) in South Carolina started looking for spots around the country where organic carbon was missing. "Initially we wanted to find volcanic terrain without organic carbon and just see what was there," Chapelle says. "The plan was to document the presence or absence of hydrogen."
At Lidy Hot Springs in Idaho, they found the perfect conditions. In this week's Nature Chapelle and colleagues reported the first discovery of an ecosystem where hydrogen-consuming microbes of Archaea rule and bacteria are so few they fall into the margin of error.
"Tell a microbial microbiologist you have an ecosystem with 99 percent Archaea and the response would be: 'That's impossible, somehow the method is wrong,'" Chapelle says. "And that's exactly what you would have to think the first time you saw it." Bacteria usually dominate because, by consuming organic carbon, they out-compete the less energetic Archaea that consume hydrogen, breathe carbon dioxide and produce methane. Chapelle worked with USGS hydrologists as well as with microbiologists from the University of Massachusetts in Amherst, Mass, to identify they types of Archaea. Three different tests confirmed that Archaea made up more than 90 percent of the microbial community. "That's really what's there," he concludes.
Penelope Boston - who with colleagues in 1992 suggested that subsurface habitats with nonorganic sources of energy could possibly fuel life on Mars - was "delighted and thrilled" to learn Chapelle had found some examples on Earth. "It makes me feel even more confident about our prediction," she says. On Mars, where rocks are bombarded by ultraviolet and ionizing radiation, life might be "tucked comfortably into the subsurface, where more moderate temperature and liquid water could exist," Boston says.
In 1995, Todd Stevens and James McKinley of Pacific Northwest National Laboratories found a similar ecosystem in the Columbia River basalts, but the microbial community only consisted of 3 percent Archaea. "When the Stevens and McKinley group discovered the methanogens in the Columbia river basalts, we felt that this was a good terrestrial example and was exactly what we predict for Mars," says Christopher McKay of NASA Ames Research Center, who co-authored the 1992 report with Boston of Complex Systems Research, Inc. "The new work by Chapelle et al. is even better. The case for methanogens being the sole source of primary production in the Lidy microbial community is quite compelling now. We feel that these terrestrial examples really make the case for deep drilling on Mars and searching for extant life there."