When Captain Robert Falcon Scott discovered the Dry Valleys as part of the British Antarctic Expedition in 1905, he described them as "valleys of the dead." But beneath their desolate, icy surface life goes on, according to a new study suggesting that microbes can remain alive for millions of years when frozen.
Previous studies of ancient ice samples taken from Antarctic subglacial lakes indicate that ice can preserve microorganisms for up to 300,000 years. In the new study, appearing online this week in the Proceedings of the National Academy of Sciences, molecular biologist Paul Falkowski of Rutgers University in New Brunswick, New Jersey, and his colleagues examined ice they had collected from the Upper Beacon Valley in Antarctica. They used a range of dating techniques, including measuring the isotope ratios in the ice, to show that samples ranged from about 100,000 years old to 8 million years old, making the latter ice the oldest sample ever studied.
After melting the samples that were 100,000 and 8 million years old, the researchers found signs of live bacteria in the water from both. Adding nutrients to the meltwater led to bacteria growth in both samples, although the microbes in the oldest ice grew much more slowly, doubling every 1 to 2 months versus every week. In another experiment examining how quickly DNA degenerates over time while frozen, the team examined any bits of DNA--whether from living or dead microbes--found within five ice samples of various ages. When the researchers compared the DNA recovered from the oldest ice sample to that from the 100,000-year-old sample, the former had DNA fragments with an average length of 210 base pairs versus 18,500 base pairs for the latter. Falkowski believes that cosmic radiation, which is particularly strong at Earth's poles, may gradually degrade the frozen DNA. If cosmic radiation indeed damages DNA at the rate indicated by the study, notes Falkowski, it challenges the idea, known as panspermia, that life on Earth was seeded by life elsewhere in the universe.
Finally, the team compared the genetic sequences extracted from the ice to known genes of modern-day bacteria. Curiously, there were few matches. That may mean the ancient microbes had novel genes. And Falkowski speculates that frozen microbes may periodically thaw and reintroduce their genes, influencing the evolution of modern microbes. "You can think of Antarctic ice as a 'gene Popsicle' that can potentially be acquired by extant organisms when the ice melts," he says.
It's a "very significant finding," says ancient DNA researcher Eske Willerslev of the University of Copenhagen in Denmark. This pushes the limits of how old DNA can be and still be recovered in an understandable form, he says. However, Willerslev cautions that as is the case with most claims of ancient microbes being revived, contamination of samples with DNA from modern microbes in the lab or elsewhere is always a possibility. "These results show patterns that you can't easily explain by contamination," he notes. "But I would feel more comfortable with the results if they had been replicated in two independent labs."