Primitive life has surfaced in one of the harshest settings yet seen: pockets of water trapped in the ice of an Antarctic desert. Single-celled organisms in the frigid bubbles barely eke out survival by drawing nutrients from sediments in the ice and from each other, researchers report in tomorrow's Science. The bare-bones ecosystem, they claim, might be the closest thing on Earth to that which could support life within the polar caps of Mars or in Europa's ice-covered ocean.
The Antarctic algae and bacteria are the latest additions to a growing list of "extremophiles," so named for their abilities to withstand seemingly inhospitable conditions. Some microbes thrive at deep-sea vents or in scalding geothermal springs, for instance, while others call home the slushy ice in polar oceans or alpine lakes. Although most such organisms multiply at healthy rates, the newly discovered microbes divide just once each year, on average--a startling case of "life on the edge," says team leader John Priscu, a microbial ecologist at Montana State University in Bozeman.
Biologists found the community within permanent ice that covers lakes in the McMurdo Dry Valleys, one of the planet's coldest and driest deserts. Most of the year, the organisms are frozen solid in the ice. But during the Antarctic summer, the sun heats dark sediment and organic matter and melts pockets, about 2 meters deep in the ice, where bacteria and blue-green algae huddle. For about 150 days, the algae photosynthesize and supply carbon and nitrogen to the bacteria, which in turn decompose organic matter and cycle carbon dioxide back to the algae. "In extreme environmental conditions, such metabolic teamwork may be necessary for survival," Priscu says. The microbes are unrelated to plankton that live in the less inhospitable waters of the lakes, he notes.
"This is the most simple habitat or symbiosis that one can imagine," says biologist Roland Psenner of Innsbruck University in Austria, who wrote a commentary about the research. The Antarctic ecosystem presents a model for how life may have arisen and persisted on other worlds, Psenner says. "That's the spectacular twist of this discovery: If organisms thrive in the hard ice of Dry Valley lakes, they may possibly have done so on Mars and Europa as well."