Life found beneath Antarctic ice sheet.

Pushing the known envelope of life to a new extreme, scientists have found evidence that viable microorganisms populate a gigantic freshwater lake hidden for hundreds of thousands of years beneath the Antarctic ice sheet.

Called Lake Vostok, the near-freezing body of water is locked in perpetual darkness beneath 4,000 meters of ice (SN: 10/2/99, p. 216). No one has yet collected samples of the trapped water, but an international drilling team has bored down to within 120 m of the lake and pulled up samples of the deep ice. According to glaciologists, ice from this level represents lake water that froze to the bottom of the glacial sheet.

"When you take this ice and melt it down, you do find some viable cells," says David M. Karl of the University of Hawaii at Manoa in Honolulu, who reported his results last week at a meeting in Cambridge, England. Originally living in the lake, these organisms became trapped in the lowermost ice as it froze, he says.

Another group, led by John Priscu of Montana State University in Bozeman, also reported observing microbes in the frozen lake water. Priscu's team, however, didn't find evidence that the cells were alive.

Karl and his colleagues used nutrients labeled with radioactive carbon to test for living microbes in the melted ice. By tracking the carbon, they demonstrated that glucose and acetate were broken down to form carbon dioxide. Living cells carry out these same reactions in a process called respiration.

"I think that was a really clever way to look at metabolism. He showed they're viable," says Priscu.

Karl notes that a similar experiment gave a positive result when it flew on the Viking spacecraft in the 1970s. In that case, researchers concluded that no organisms were involved but extreme ultraviolet radiation hitting Mars had broken down the radioactively labeled molecules into carbon dioxide. However, the Hawaiian team performed its Vostok experiment in the dark and conducted several control tests to rule out false-positive results, says Karl.

At this point, neither team can identify the microorganisms. About 1 micrometer in size, they could be either bacteria or members of a domain of life called archaea, which populates many extreme environments. Priscu's team is currently trying to sequence DNA isolated from the ice and match it against known organisms. "That will tell us if they are ancient or unique," he says.

Whatever microbes are present in the deep ice must be mini-Methuselahs. Both Priscu and Karl used radioactively labeled compounds to test whether the cells were growing, a different process from respiration. Neither group could find evidence that the organisms bulked up or reproduced. "These cells are probably growing very, very slowly," says Karl.

A French team also tested the ice but couldn't demonstrate that the cells they found were originally from the ice rather than contaminants introduced during drilling or lab studies.

To confirm the presence of living microbes in the lake itself, scientists must punch through the ice sheet and collect samples of the water, says Karl. Such a mission would also help determine how organisms can survive in such a starved environment. Some geologists have suggested there may be hot springs at the bottom of the lake spewing out nutrients. At the Cambridge meeting, researchers discussed mounting an international mission to breach the lake without contaminating the water.

That's going to be a difficult task, says Tommy J. Phelps of Oak Ridge (Tenn.) National Laboratory, who studies bacteria living within Earth's crust. "I would hate for my grandchildren to blame our generation for perturbing the lake," he says.