Bacteria under ice: Some don't like it hot.

Hordes of bacteria can thrive in the seemingly inhospitable environment underneath glaciers, a region formerly considered devoid of much biology. This finding by glaciologists working in Switzerland could help solve some puzzles of the last ice age and point the way for finding life on other planets.

Bacteria with odd lifestyles have come under increasing scrutiny of late, with most research focused on the so-called thermophilic species, which prefer scalding homes. The new study shifts attention to the other end of the thermometer. "The exciting thing is the idea of pushing the window of acceptable microbial environments a little bit farther open," comments William H. Schlesinger, a biogeochemist at Duke University in Durham, N.C.

Researchers have previously collected small numbers of bacteria from ice in Antarctica and Greenland, but they could not determine whether these were active microbes or just frozen cells blown in by wind. In contrast, sediments beneath two Swiss glaciers harbor large colonies of bacteria--hundreds of millions of cells per gram of gravel--that appear to be growing at 0 [degrees] C, according to a report in the February GEOLOGY.

Martin Sharp of the University of Alberta in Edmonton and his colleagues from the United Kingdom began searching for cold-loving bacteria after finding that something was causing chemical reactions beneath the ice of a Swiss glacier. At certain times of year, nitrate showed up in water flowing into the glacier but not in the water exiting from beneath it. In other environments, bacteria are known to break down nitrate.

Sharp and his coworkers followed up on these findings by taking samples of ice, water, and sediment at two mountain glaciers. Sediments beneath the glaciers contained much larger populations of bacteria than did surface or interior ice, the researchers report.

That finding indicates that the bacteria were growing at the bottom of the glacier and are not contaminants washed in while the scientists drilled through the ice, says Sharp.

Looking at the bacteria under a microscope, the researchers found that many were in the process of dividing, providing further evidence that the microbial colonies are alive and healthy under the ice. The bacteria might break down minerals and plant remains originally buried beneath the glacier or subsequently washed in by water percolating through the ice, says Sharp.

This possibility intrigues climate researchers who can't determine the whereabouts of more than 150 billion tons of carbon during the last ice age, which peaked 20,000 years ago. Sharp and his colleagues suggest that the northern ice sheets blanketed large quantities of carbon-rich vegetation in the forests and tundra. Glaciologists had formerly rejected this idea because they could find no sign of the pre-ice age forests, but the new results suggest that bacteria could have slowly decomposed such organic matter, Sharp says.

"Some of the assumptions we have made in the past now must be seriously questioned," says Richard B. Alley, a glaciologist at Pennsylvania State University in State College.

If bacteria can live underneath glaciers on Earth, why not on other planets? The new study, says Sharp, "points out in many ways that the bottoms of glaciers are probably quite good environments from the point of view of bacteria. So, maybe looking at the bottom of the Martian ice sheets would be a sensible place to try if you're going to look for life on Mars."