Image caption Researchers have to drill through thousands of metres of ice to reach the surface of Lake Vostok

There could be some complex animals living in Lake Vostok, which lies close to 4km below Antarctica's ice sheet.

The possibility is raised by scientists who have sifted genetic material in ice drilled from close to Vostok's surface.

They found signatures for organisms such as bacteria that are often associated with marine molluscs, crustaceans and even fish.

But the team cautions in the PLoS One journal that this material may also represent past contamination.

Scientists now recognise that Antarctica is underlain by a complex network of rivers, and many of the identified organisms, or their traces, could perhaps have been delivered to Vostok from the ocean. The lake is 200m below sea level.

It is, nonetheless, another fascinating twist in the story of this deeply buried lake.

First identified in 1956 by the Russians and mapped in the 1990s by the British, Vostok covers an area of 15,000 square km, and in places is 800m deep.

Researchers believe it has not been open to the atmosphere for many millions of years, and a drilling effort has recently tried to sample its waters.

The new PLoS study examined genetic material - stretches of RNA - isolated from ice that froze on to the ice sheet as it moved above the lake. The supposition was that this content might hint at the type of life present in Vostok.

Thousands of unique matches were identified with sequences already listed in public databases.

The vast majority (94%) of these matches were with bacteria, while a smaller group (6%) were with more complex, multi-cellular organisms (eukaryotes). A handful of links were made also to archaea - very primitive, single-celled microbes.

A large number of bacterial sequences, reports the team, were from "animal commensals, mutualists and pathogens… including those associated with annelids, sea anemones, brachiopods, tardigrades and fish."

The team also found matches to types of bacteria that thrive in hot environments, such as around volcanic hydrothermal vents on the sea floor. If such vents existed in Vostok, they could "provide sources of energy and nutrients vital for organisms living in the lake", the team writes in PloS One.

Lake Vostok is the largest of about 375 sub-glacial bodies of water now mapped under Antarctica's ice sheet.

These "ghost" lakes are kept in a liquid state by heat rising from the rockbed below and from the pressure of all the ice pushing down from above.

Astrobiologists have a particular interest in the lakes.

Conditions in them may not be that different from those in the liquid water bodies thought to exist under the surfaces of icy moons in the outer Solar System.

Places like Europa, which orbits Jupiter, and Enceladus, which circles Saturn, may be among the best places beyond Earth to go to look for alien organisms.