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Life at the Bottom

9 December 1999 7:30 pm
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Lake Vostok, visible from space despite 4 kilometers of ice.

Biologists have found living bacteria at the bottom of the deepest ice core ever drilled. The discovery gives a first glimpse of how life may have survived in an ecosystem cut off from the rest of the world for a million years Vostok Station, a Russian research outpost in the heart of Antarctica, sits atop a glacier nearly 4 kilometers thick. Amazingly, under all that ice lies a lake of liquid water, called Lake Vostok, that is 700 meters deep and as big in area as Lake Ontario. Geologists think Vostok might be warmed by geothermal energy, but they don't know for sure because they have not yet penetrated down to the lake. In 1998, a Russian-French-U.S. team drilled to within 100 meters of the surface before stopping to avoid contaminating what one researcher calls "the last pristine water on Earth."

Since then scientists have been studying samples of the bottom core for clues to what might lie beneath the ice. A team led by Jean Jouzel of the Centre Nationale des Recherches Scientifiques in Gif-sur-Yvette, France, reports in tomorrow's issue of Science that the end of the core is frozen lake water, not water that arrived in Antarctica as precipitation. The most convincing evidence for this, says Jouzel, comes from isotope ratios; ice that has frozen in site has a higher proportion of water molecules containing the heavy form of oxygen, oxygen-18, than that of ice that has been transported over long distances by weather systems.

Two other teams, led by David Karl of the University of Hawaii, Manoa, and John Priscu of Montana State University in Bozeman, found "coccoid," or spherical, bacteria in the lake ice, and Karl's team managed to revive the bacteria in his lab. The microbes were unusually small--a sign, Karl says, that they were starving. "The bacteria are probably not growing in the ice, because water is not available," Karl says. The number of cells found was also about 10 times less per cubic centimeter than in even the harshest habitats found elsewhere on Earth.

Priscu's group found a higher concentration of cells, possibly because their core sample included particles that might be rock flour from the rim of the lake. From DNA tests, they tentatively assigned the bacteria to four groups that were previously known to survive in extreme environments.

Karl says it's too early to say whether the frozen organisms are representative of life in the lake itself. Life requires energy, and if the only sources of energy are ice melt from the glacier above and the minimal energy from the crust necessary to keep the lake liquid, the pace of life in the lake could be slow indeed. On the other hand, if the lake is warmed from below by ample geothermal heat, then there could be many more kinds of organisms, possibly even a food web. At present there is no sign of that: The ice does not contain any viruses or bacterial predators, such as protozoans. "It's a big debate," Karl says. "We need to penetrate and get a lake sample, that's the bottom line."

Other scientists warn, though, that such a step cannot be taken lightly. "We have an opportunity to make a million-year mistake," says Warwick Vincent of Laval University in Quebec. The question of when and how to send a probe into the lake itself is still under discussion (Science, 1 October, p. 36).

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