Kay Bidle, assistant professor of marine and coastal sciences at Rutgers University, said: "The young stuff grew really fast. We recovered them easily... They doubled every couple of days."

By contrast, microbes from the oldest samples grew slowly, doubling only every 70 days.

The findings were reported in the online edition of the Proceedings of the National Academy of Sciences journal.

The research found that the DNA in bacteria deteriorates sharply after about 1.1 million years. Bidle said that after 1.1 million years the size of the DNA gets cut in half.

In the oldest ice it consisted of just 210 units strung together. Normally the DNA of the average bacterium has about three million units.

Studying the organisms helps in "understanding the geological and physiological limits of life on Earth under different conditions," Bidle explained in an interview with the Associated Press news agency.

"They live in every possible environment ... so learning about microbes and what they can withstand and what their limits are is important to understanding how the Earth works over long periods of time," he said.