Several life forms have been discovered living and reproducing in a lake buried under 3.5 kilometres of ice in Antarctica, scientists say.

"We found much more complexity than anyone thought," said Scott Rogers, a professor of biological sciences at Bowling Green State University in Ohio, whose team led the investigations in Lake Vostok.

"It really shows the tenacity of life, and how organisms can survive in places where a couple dozen years ago we thought nothing could survive."

In research published June 26 in the Public Library of Science journal, Rogers said the presence of marine and freshwater species supports the hypothesis that the lake once was connected to the ocean, and that freshwater was deposited in the lake by the overriding glacier.

By sequencing the DNA and RNA from ice samples, the team identified thousands of bacteria, including some that are commonly found in the digestive systems of fish, crustaceans and annelid worms, in addition to fungi and two species of archaea, or single-celled organisms that tend to live in extreme environments.

They also discovered psychrophiles, or organisms that live in extreme cold, along with heat-loving thermophiles, which Rogers said suggests the presence of hydrothermal vents deep in the lake.

Lake Vostok is the fourth-deepest lake on Earth and the largest of about 400 subglacial lakes known in Antarctica. The ice that has covered it for the past 15 million years is now more than three kilometres deep, creating tremendous pressure in the lake. Few nutrients are available.

Connection to Southern ocean

The lake lies far below sea level in a depression that formed 60 million years ago, when the continental plates shifted and cracked. The weather there is so harsh and unpredictable that scientists visiting must have special gear and take survival training. It's been widely believed that the lake is completely inhospitable to life.

More than 35 million years ago, Antarctica had a temperate climate and was inhabited by diverse plants and animals. About 34 million years ago, a huge drop in temperature occurred and ice covered the lake, when it was probably still connected to the southern Atlantic Ocean, Rogers added.

This lowered the sea level by about 90 metres, which could have cut off Lake Vostok from the ocean. The ice cover was intermittent until a second big plunge in temperature took place 14 million years ago, and the sea level dropped even farther.