Forever young?: Researchers have probed the secrets of how bacteria are able to persist in 750,000-year-old Siberian permafrost (pictured here under the microscope).
Credit: Rachel Sussman
SYDNEY: DNA repair could be the long sought after secret that has allowed bacteria to survive for hundreds of thousands of years in permafrost, say experts in the U.S. journal Proceedings of the National Academy of Sciences.
Many research teams have made recent claims that they discovered bacteria still living after thousands or even millions of years of being locked inside arctic permafrost, amber or underground salt crystals.
"However, these claims have suffered from two drawbacks," said Eske Willerslev, from the Ancient DNA and Evolution Group at the University of Copenhagen in Denmark.
Firstly, the studies have not included appropriate controls to exclude modern day bacteria from contaminating them, he said, and secondly, the studies have not shown how the bacteria might avoid death through spontaneous damage to their genomes.
Dormant state
The best theory for how the prehistoric microbes survived has been that they entered a state of dormancy, which some microbes are known to use to battle through extreme conditions.
However, true dormancy is characterised by the absence of any metabolic activity, including the DNA repair enzymes which normally correct damage caused by the chemistry of metabolism. Without a continuously active DNA repair system, any cell would be expected to accumulate fatal damage over these lengthy time spans.
Now Willerslev and his team think they have the answer to how the bacteria can side step this cumulative damage. In order to answer the question, they analysed permafrost samples taken from Siberia, Canada and Antarctica ranging in age from 7,000 to one million years old.
First, the researchers took advantage of the fact that strands of DNA extracted from dead organisms are generally much shorter than those that can be extracted from living organisms. DNA that appeared to come from living bacteria was found in samples as old as 740,000 years. The results were replicated by Michael Bunce from Western Australia's Murdoch University, making this the oldest independently verified DNA obtained from living cells.
Understanding ageing
The team then studied this DNA for signs of damage (a common kind of mutation called a 'reversion'), consistent with a lack of reparative enzymes. The DNA from one type of bacteria that was dated to between 5,000 and 35,000 years old showed signs of DNA damage that suggested a dormant state. However, surprisingly, the DNA from another kind of bacteria that had persisted for over half a million years showed a clear signature of DNA repair.
The team separately examined the permafrost samples for carbon dioxide production – a symptom of metabolic activity – and found that samples up to 600,000 years old produced a steady, if minute, stream of the gas. In short, "some of the bacteria are indeed metabolically active and able to repair their DNA," said Willerslev, "thus actively dealing with spontaneous decay of their genomes."
It's not yet clear why the older bacteria appear to be more metabolically active than those from younger samples – or how they are to survive if they are not completely dormant. However the researchers found evidence that the younger DNA was from a spore-forming bacteria – possibly explaining the dormancy – while the older DNA was not.
The authors suggest that permafrost around the world may harbour species of viable bacteria adapted to past environments. They also suggest that if Mars or Europa, one of Jupiter's moons, was ever home to life similar to bacteria, they too may be still alive in the permafrost.
According to Willerslev, the real mystery is why some cells live longer than others. "What exactly makes bacterial cells able to survive for up to half a million years while others – for example human cells – cannot survive anywhere near that length of time? Understanding that may be a key to understand ageing," he said.
