Microbes thrive below Antarctic ice. A hint of life elsewhere in solar system?
The discovery of bacteria in an ice-bound lake bolsters the case that similar life could exist elsewhere in the solar system. But on Earth, the find raises the prospect that Antarctic melting will release greenhouse gases.
Researchers have uncovered a thriving community of microbes in a lake some 2,600 feet below the surface of the West Antarctic Ice Sheet, the first direct evidence of life in such lakes and the first window on the ecosystem the microbes occupy.
Genetic evidence extracted from samples of lake water indicates that the lake teems with a wide variety of microbes that make up a complete food chain, with those at the bottom drawing their energy from chemicals in rocks and sediment in the lake bed.
In essence, the community is a frosty variation on the microbial communities that form the bottom of the food chain around hydrothermal vents in the lightless depths of the ocean floor.
The discovery confirms a long-standing expectation that life could thrive in this type of extreme environment. But "could" is not the same as "does." It's a difference with important implications.
These organisms "are active; they have a function," says Jill Mikucki, a microbiologist at the University of Tennessee in Knoxville and a member of the international team reporting the discovery in Thursday's issue of the journal Nature.
Indeed, they are a poorly understood part of the global biogeochemical cycle, she suggests.
"About 10 percent of the Earth's terrestrial surface is covered by ice. We know very little about the chemical and biological activities that are going on in that sub-ice environment," she says. Yet through their internal chemical reactions, these organisms "are doing things like liberating nutrients from rock material, which can influence productivity in the world's oceans."
The activity of these communities in Antarctica bears on at least two other issues as well: global climate and the prospects for finding life on Jupiter's moon Europa, Saturn's moon Enceladus, or even on Mars, other researchers add.
Astrobiologists have looked at Enceladus and Europa as prime candidates for hosting at least microbial life. Both moons have oceans underneath their ice crusts. The results from Antarctica will only bolster their case.
Even Mars' stock as a potential host for extant life may have risen, suggests Martyn Trantor, a glaciologist at the University of Bristol in Britain. He notes that Mars has ice sheets. The idea that microbes could be lurking beneath them "has more traction now," he wrote in a commentary in Nature tied to the new results.
As for Earth's climate, researchers have speculated that if microbial communities can thrive under the ice sheet, they could be contributing to the buildup of large reservoirs of methane beneath the West Antarctic Ice Sheet in the organic-rich sediments of ancient coastal wetlands and marine basins.
"Potentially, that methane could be released to the atmosphere" as the West Antarctic Ice Sheet retreats with global warming, says Martin Sharp, a glaciologist at the University of Alberta in Edmonton. Molecule for molecule, methane is a more potent greenhouse gas than the more abundant atmospheric carbon dioxide.
"That becomes a much more realistic possibility given this finding," says Dr. Sharp, a pioneer in uncovering biological activity beneath glaciers. He did not take part in the work in Antarctica leading to the discovery.
"It's a great study and an incredible achievement to have been able to do this," he says of the project, dubbed the Whillans Ice Stream Subglacial Access Research Drilling. The effort was led by Montana State University ecologist John Priscu.
Based on results from studies of small glaciers outside of Antarctica, researchers had come to expect that microbial life also could lurk in waters thousands of feet below the continent's vast ice sheets.
One target for the search: Lake Vostok, the continent's largest under-ice lake. Lake Vostok is located under the central East Antarctic Ice Sheet.
In July 2013, for instance, researchers working at the Vostok site reported evidence for bacteria in core samples taken from ice that had built up just above the lake's surface.
The team took pains to remove potential sources of contamination from the core samples before they analyzed them – a problem that had plagued the analysis of data from earlier attempts to detect life in Lake Vostok. But the 2013 evidence, described in the journal PLoSOne, remained indirect rather than being drawn from the lake itself.
The ice was frozen lake water. It yielded important insights, researchers say. But in the act of freezing, water could have ejected evidence that might have had a bearing on how the Vostok team interpreted its results. The researchers themselves ended their analysis by saying that their lines of evidence "suggest that viable organisms exist in the lake water," and that the lake "might contain a complex web of organisms, zones, and habitats" tens of millions of years old.
The new results come from Lake Whillans, which lies under the West Antarctic Ice Sheet about 1,000 miles from Lake Vostok. It sits beneath the Whillans ice stream, which is slowly making its way to the Pacific sector of the Southern Ocean. The lake is about 60 miles inland from the point where the Ross Ice Shelf becomes a floating expanse of ice. The lake is part of a chain of under-ice lakes, themselves part of a submerged coastal watershed that emptied into what once was a large bay.
Unlike Vostok, where it takes 13,300 years for water entering the lake to leave it, Lake Whillans and other lakes like it refresh themselves on time scales of years to decades, Dr. Priscu's team explains.
The team used a hot-water drill to reach the lake, went to meticulous lengths to ensure that the water in the drill, the drilling and sampling gear, and the people running the drilling operation were as free of contamination as possible. Members of the team handling the gear or running the drill wore protective clothing. The drill water was heavily filtered, pasteurized, and exposed to intense ultraviolet light. The drilling gear also got the ultraviolet treatment as well as a wash-down with a hydrogen-peroxide solution.
The researchers found nearly 4,000 types of bacteria, explains Trista Vick-Majors, a PhD candidate at Montana State University and a member of the research team. Some served as primary producers, taking in carbon dioxide while in effect feeding on minerals in rocks and sediment – iron or sulfur, for example – or taking up nitrogen in the form of ammonia, which seeps up from decomposing organic material in the sediment.
"Those guys are doing the heavy lifting for the ecosystem," Ms. Vick-Majors says. Others fed off of the organic matter these primary producers make.
Indeed, the system seems to have its own nitrogen recycling program, where primary producers convert ammonia from the sediments into organic nitrogen that others can feed on. Those others, in turn, produce ammonia as waste, which they eject and which the primary producers can use.
"It's a way of getting around being isolated from the outside world," she says. "You carefully recycle what you have."
The results, which included an analysis of sediment samples taken from the lake bed, some seven feet below the lake surface, represent "the first definitive evidence that there's not only life, but active ecosystems underneath the Antarctic ice sheet, something that we have been guessing about for decades," said the paper's lead author, Brent Christner, in a prepared statement. Dr. Christner is a microbiologist at Louisiana State University in Baton Rouge and the lead author of the paper reporting the results.