An improbable microbe, first found living 3.3 kilometres below South Africa, has been glimpsed half a world away in California.
Candidatus Desulforudis audaxviator was found in water-filled fractures in South Africa's deep gold mines, where it lives isolated from every other life form on Earth.
Uniquely, the bacterium has evolved to do without the sun's energy, relying only on hydrogen and sulphate, which can form naturally in its subterranean home.
Until recently, biologists thought that the species was confined to South Africa's depths – although related DNA has shown up in Europe and in igneous crust below the sea floor.
Now, a project to map Earth's deep biosphere – the Census of Deep Life – has found DNA 99 per cent identical to that of D. audaxviator tens of thousands of kilometres away, in boreholes 900 metres deep near Death Valley in eastern California.
Duane Moser at the Desert Research Institute in Las Vegas, Nevada, discussed the find at the American Geophysical Union conference in San Francisco on 6 December. "We're reasonably sure we're looking at the same bug," he says.
Life can find a way
If so, D. audaxviator – literally, "bold traveller" – merits its name. If the bacterium came from South Africa, it probably started out at shallow depths and evolved to live in ever-greater isolation as it descended, says Moser. But then some bacteria may have made the return journey and reached the surface, perhaps through water springs. From there they may have surfed the winds before raining down on the US and begun to descend once more.
The scenario is certainly possible, says Tullis Onstott at Princeton University, who has worked extensively on the South African D. audaxviator populations.
Alternatively, the horizontal flow of water underground may allow deep microbes to colonise new regions without returning to the surface, says Jan Amend at the University of Southern California in Los Angeles. The move between South Africa and the New World must have happened before the Atlantic opened between them, though.
"We're taught in school that all life needs some input from the sun," says Moser. "What we're seeing in D. audaxviator is that even where the sun hasn't shone for hundreds of millions of years – like the interior of Earth or Mars – life can find a way."
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World's loneliest bug turns up in Death Valley
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World's loneliest bug turns up in Death Valley
