What is excluded is that there is some form of life, like that found in the deep ocean

Philippe Jean-Baptiste

But an analysis of ice drilled from just above the lake has found no evidence for the existence of superhot springs, which sustain life on the sea floor.

There is, though, a possibility that bacteria could still be found elsewhere in the lake, says the US-Russian team behind the discovery.

"It does not exclude that there is some form of life inside the lake in terms of bacteria or microbes," said Philippe Jean-Baptiste, of the Laboratoire des Sciences du Climat et de l'Environnement, Saclay, France.

"But what is excluded is that there is some form of life like that found in the deep ocean."

Strange animals

Lake Vostok, a body of fresh water about the size of Lake Ontario, is arguably the most isolated aquatic environment on Earth. It is the largest of over 70 sub-glacial lakes on the white continent and has attracted intense interest from researchers.

The lake was cut off from the rest of the biosphere at least a million years ago, and this could have provided a unique opportunity for life in the water to develop along a separate evolutionary path.

Satellite image of Lake Vostok

They have been trying to establish whether the lake's bed might contain hydrothermal vents, through which warm chemical-rich water can flow.

Where these vents occur on the ocean floor, they support a variety of strange animals capable of living in the absence of sunlight, such as giant tubeworms.

"There was a suspicion that this could also happen in lakes like Vostok," Dr Jean-Baptiste told BBC News Online. "From our measurements, we say that this is not the case."

Deep sealife

The conclusion, reported in the scientific journal Nature, is based on a chemical analysis of ice drilled from the Antarctic lake.

The ice was sampled at a depth of more than 3,500 metres (3,800 yards), the boundary between glacial ice and frozen lake water.

John Priscu holding an ice core from an Antarctic lake

Measurements of isotopes (different types of the same atom) for the atmospheric gas helium suggest that there is no energy input from vents.

"The lack of helium within the ice suggests that there is no flow of water or volcanic material from deep within the Earth," said a rival researcher, Robin Bell of Columbia University, New York.

But she said she thought some form of life would still be found in the lake.

"I don't think there are going to be tubeworms but there could be bacterial colonies associated with hotsprings coming from something other than volcanoes," Dr Bell told BBC News Online.

Life origins

Hydrothermal vents form when seawater seeping into cracks in the Earth's crust becomes superheated by molten rock.

The water takes minerals from the rock, rises and bursts out into the ocean through holes in the seafloor.

Bacteria process the inorganic chemicals in the hot fluids to begin a food web that can eventually support more complex organisms like shrimps

Some scientists believe that life on Earth first formed in a hydrothermal vent system about 3.5 - 3.8 billion years ago. There is even a possibility that vents could be harbouring life on one of Jupiter's moons, Europa. Pulled about by the gas giant's huge gravitational influence, the satellite is thought to have a liquid ocean under its ice-covered surface.

This has prompted the American space agency (Nasa) to take a keen interest in Lake Vostok for what it might tell us about extra-terrestrial life.