Repairing DNA could let frozen bacteria survive for millennia

An international team of scientists believe they have strong evidence that bacteria trapped in permafrost are able to survive for hundreds of thousands of years by repairing their DNA.

If Eske Willerslev of the University of Copenhagen, Denmark and his colleagues are correct, then the findings could mean that the frozen poles of Mars and Jupiter’s moon Europa could also harbour ancient life. But experts on microbial survival contacted by Chemistry World were split on the significance of the findings, published in the Proceedings of the National Academy of Sciences  this week¹.

Accepted wisdom is that to survive extremely cold conditions bacteria shut down their metabolism and stop cellular processes such as DNA repair - leaving them vulnerable to genetic damage by process like deamination and oxidation. If the microbes stay frozen for long enough, the damage breaks up their DNA and kills them off.

But when Willerslev and his team drilled into Siberian permafrost, frozen solid for over half a million years, they found bacteria with long stretches of undamaged DNA. ’This . is coupled with the fact that there was CO₂ release from the samples’ - showing the microbes were alive and breaking down food to produce energy, he told Chemistry World.

Nikolai Panikov of Dartmouth College, Hanover, US, said Willerslev’s paper seems to support other recent data which suggest microbes stay active and even reproduce at very low temperatures.

The microbes use trace levels of volatile organic compounds such as ethanol, methane and hydrogen that diffuse through the frozen soil. ’The cells can absorb and oxidize them to generate energy for maintenance and even slow growth,’ Panikov said. He was not convinced, however, that the carbon dioxide measurements made by Willerlev’s team were reliable enough to prove the microbes they found were active.

Russell Vreeland of West Chester University, Pennsylvania, US, who claimed in 2000 to have discovered 250 million-year-old bacteria in salt crystals, also rejected the findings. The bacteria would have to be cultured to prove outright that they were viable, Vreeland said.

But Richard B Hoover of NASA’s Marshall Space Flight Center says Willerlev’s conclusions are ’very solid.’ ’I am delighted to see that more and more people are coming round to the idea that bacteria can survive in these environments,’ he told Chemistry World.

Hoover’s own 2005 claim to have identified a new species of bacteria in 32,000 year-old ice was met with scepticism by other scientists. He says a lot of preconceptions about organisms that live in extreme conditions are now being challenged. ’This paper is one step in this revision’ he said.

Tom Westgate