Published online 22 May 2008 | Nature | doi:10.1038/news.2008.850

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How low can life go?

Microbes thrive more than a kilometre beneath the sea floor.

ocean floorThe ocean floor is not the limit for life.Punchstock

Microbes have created a home in 111-million-year-old rock buried 1.6 kilometres below the sea floor, researchers have found.

The discovery, published today in Science1, beats the old record — 842 metres below the sea floor — but may not stand for long. Some experts think that microbes could potentially set up home as far down as 5 kilometres below the sea floor.

Microbes have been found in nearly every nook and cranny that Earth has to offer, from deep-sea vents to the drainage from acid mines. In 2002, John Parkes, now at the University of Cardiff, UK, and his colleagues discovered bacteria in porous rock more than 800 metres beneath the bottom of the Pacific Ocean2.

Some estimates reckon that two-thirds of Earth’s microbial biomass could be found below the sea floor — although given the difficulty of sampling so far below the waves, it is almost impossible to say for certain.

Under the sea

But conditions become progressively harsher deeper in the sediment. The rock becomes older and more likely to be depleted of the organic material for microbes to feast on. Meanwhile, pressure and temperature steadily rise. In some regions the temperature rises 20 °C for every kilometre deeper below the sea floor.

At present, the uppermost temperature at which life can survive is estimated at around 120 °C. “If temperature is the ultimate limit, then one might reasonably expect the biosphere to extend as much as 5 kilometres below the sea floor,” says Steven D’Hondt, an oceanographer at the University of Rhode Island.

JOIDES ResolutionThe samples were collected by the research vessel JOIDES Resolution.Integrated Ocean Drilling Program, Texas A&M University

The new sample was retrieved from the Newfoundland Margin in the Atlantic Ocean, by the ocean drilling ship JOIDES Resolution. The ship, once used to drill for oil, was refitted with scientific equipment more than 20 years ago and has since drilled thousands of cores around the world.

Parkes and his colleagues extracted the microbes from the inner core of the sediment samples, where they were unlikely to have been contaminated with external seawater. They found evidence of living microbes by staining the samples with a dye that fluoresces green when it enters a live cell.

The researchers were also able to isolate DNA from the microbes. DNA sequences indicated that several species of Archaea, mainly members of the heat-loving genus Pyrococcus, lived 1,000 metres deep. As depth and methane concentrations increased, they found additional DNA sequences from microbes that oxidize methane to produce energy.

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How the microbes find food in such old rock remains a mystery, says Parkes. “They are still making a living in material that is millions of years old,” he says. “Anything that is at all degradable you would have thought would be long removed.” Parkes speculates the microbes get by with very little food. With no predators to dodge, the microbes could just subsist on enough energy to eek out a molecule of ATP from time to time, he says.

Meanwhile, for Parkes the next step in his research is clear: “We’d love to go deeper,” he says - perhaps as much as 6 kilometres in the hope of seeing life at the very limit. 

  • References

    1. Roussel, E. G. et al. Science 320, 1046 (2008). | Article | PubMed | ChemPort |
    2. Wellsbury, P., Mather, I. & Parkes, R. J. FEMS Microbiol. Ecol. 42, 59–70 (2002).
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