WASHINGTON — In ice that has sealed a salty Antarctic lake for more than 2,800 years, scientists have found frozen bacteria and algae that returned to life after thawing. The research may help in the search for life on Mars, which is thought to have subsurface lakes of ice.

A research team led by Peter Doran of the University of Illinois at Chicago drilled through more than 12 metres of ice to collect samples of microscopic bacteria and algae.

"When we brought them back and warmed them up a bit, they sprang back to life," said Doran, the first author of a study appearing this week in the Proceedings of the National Academy of Sciences.

Doran said the microbes have been age-dated at 2,800 years old, but even older microbes may live deeper in the ice sheet sealing the lake, and in the briny water below the ice. That deeper ice and the water itself will be cautiously sampled in a later expedition that will test techniques that may one day be used on Mars.

Called Lake Vida, the eight-square-kilometre body is one of a series of lakes located in the McMurdo Dry Valleys of Antarctica, some 3,200 kilometres south of New Zealand.

"This lake has been known since the 1950s, but people ignored it because they thought it was just a big block of ice," said Doran. While at the site for other research in the 1990s, Doran and his colleagues sent radar signals into the clear ice covering the lake and were surprised to find that about 20 metres below there was a pool of liquid water that was about seven times more salty than seawater.

That prompted the researchers to return in 1996 with equipment to drill a hole down to within a few metres of the water layer. At the bottom of this hole, they harvested specimens of algae and bacteria.

Doran said the researchers halted drilling at 12 metres to preserve an ice shell over the surface of the ancient lake.

"We haven't broken the ice seal (over the water) yet," said Doran. "We don't want to disturb that environment until we're ready."

The researchers will return in 2004 equipped with instruments that are sterilized. They will then drill through the full 20 metres of ice and sample some of the briny water from the lake for analysis. The water specimen will be cultured to see if it contains life.

Specimens from the water are expected to be even older than the life forms extracted from the ice covering.

Doran said besides making sure the equipment is germ-free, the researchers will suck up samples of the water into a sealed airlock.

"We don't want to contaminate the lake," he said. "If we find life in the lake, we want to make sure it was living in the lake and not on our instruments."

The new study is being funded by the National Aeronautics and Space Administration because the space agency is anxious learn how best to search for life in submerged pools of briny water, said Doran, noting that Lake Vida may be an Earthly version of water deposits thought to exist on Mars.

"There is evidence of flow features on Mars made within the last million years," said Doran. "Some kind of fluid seeping out. One theory is that it could be pockets of brine like the one in Lake Vida that will not freeze because they are so salty."

Doran said water flows into Lake Vida during some Antarctica summers, but the new water stays on top of the ice, sealing the lake.

"In warm summers, you get a lake on top of a lake -- a one metre of water sitting on top of 19 metres of ice," he said. "That freezes over the winter and you get a buildup year after year."

Salt is expelled as the water turns to ice and is concentrated below, said Doran. Over many thousands of years, the salt concentrates in the water, making the water increasingly difficult to freeze.

"If we do find life in that water, it will be from one of the most extreme environments on Earth," said Doran. He noted the water is not exposed to the air, receives no sunlight, has a very low temperature and a high salt content.

For life to exist there, he said, it would have to get its energy from chemicals in the water, not directly or indirectly from sunlight as do most known life forms.

Searching for such extreme life forms in Lake Vida, said Doran, will be good practice for humans who one day will search for evidence of life on Mars.

"I see Lake Vida as being like life's last romp on Mars, potentially in an ice-sealed lake," he said.