Ancient Bacteria Survive Frozen Eons

Aug. 28, 2007 — Scientists say they can at last explain how bacteria stay alive in ice for hundreds of thousands of years.

Bacteria slow down their metabolism to the point where they produce just enough energy to repair their aging DNA, say Australian researcher Mike Bunce of Murdoch University in Perth and his international colleagues.

The researchers publish their findings today in the Proceedings of the National Academy of Sciences.

"We're looking at bacteria that have survived in the permafrost for hundreds of thousands of years. These aren't dead; these are viable cells," said Bunce, a molecular biologist.

"If the DNA wasn't being repaired the bacteria would accumulate too much damage to their genome and the cells just would not be viable."

The researchers collected bacteria in soil samples from the Siberian permafrost, put the samples in sealed containers and detected the release of carbon dioxide.

"You can see they are actually respiring," said Bunce. "Samples up to about 600,000 years actually showed viable bacterial populations."

The researchers also extracted intact strands of bacterial DNA, which also supports the idea that they are alive.

"The longest ancient DNA study that's ever been produced got 1,000 base pairs of DNA from a dead specimen," says Bunce. "We're getting 4,000 base pairs."

Scientists already know bacteria can survive for many years in harsh conditions by switching off their metabolism, and protecting themselves in a hard coat called an endospore.

But during such dormancy their DNA slowly gets damaged by radiation and other factors.

Because the bacteria aren't generating any energy they can't repair the DNA and over time they just die.

So, how could the Siberian bacteria still be respiring after more than half a million years?

A closer look at the DNA shows telltale signs the bacteria are repairing their DNA, said Bunce.

"We can actually get very big pieces of DNA and it's very undamaged, which is indicative of the fact that they've got a constant level of DNA repair going on in their cells," he said.

"That's really the key to the survival mechanism for thousands of years in ice."

Bunce says the respiration experiment may be useful for people trying to detect life on other planets.

Over the years many scientists have said they have discovered ancient DNA but they have often been criticized for not ruling out contamination with more recent material.

To counter this, Bunce and colleagues used a special method to collect their samples. They also confirmed their DNA analysis by reproducing it in a separate laboratory.

Mike Dyall-Smith of the University of Melbourne studies the survival of microbes in another harsh environment, salt lakes, and is impressed with the work.

"It looks pretty good," Dyall-Smith said.

While older living bacteria have been reported, Dyall-Smith said this is the first experiment to show evidence of respiration and DNA repair in permafrost samples.

But he said he'd like to see more detail to ensure the carbon dioxide being detected is from the respiring bacteria. Experiments that show how respiration varies with temperature and stops when the microbes are killed would be useful.

"It's interesting stuff," said Professor Alan Cooper of the University of Adelaide, an expert in ancient DNA.

But he cautions the evidence is indirect since the researchers studied the soil samples as a whole, rather than the bacteria directly.

The ultimate proof that these bacteria are alive would be if the researchers could culture them.

It is not clear whether the researchers attempted to do this. But they say in their paper that it's believed that less than 1 percent of all bacterial cells can be cultured using standard methods.