BioEd Online

Sediments that lie hundreds of metres below the ocean floor contain living bacteria, an analysis has shown. And according to the study these microbes could produce significant amounts of methane, a gas implicated in global warming.

Marine sediments cover about 70% of the Earth and harbour more than half of the microorganisms on the planet. Unable to discriminate between living and dead cells in the deep ocean floor, scientists have long debated how much of its biological content is active.

To solve this riddle, a team of researchers set out in 2001 to take samples from below the Pacific Ocean with the support of the Integrated Ocean Drilling Program. They collected sediments that were up to 16 million years old from 400 metres under the floor.

The group then ran tests on the samples to detect the presence of ribosomal RNA from bacteria. As a genetic molecule that breaks down quickly after being produced, this RNA is a sign of living cells, and the tests found substantial quantities of it.

“We didn’t have clear evidence that bacteria there were alive until now,” says ecologist Lev Neretin of the Max Planck Institute for Marine Microbiology in Bremen, Germany, who was a member of the team. He and his fellow researchers estimate that, overall, about 10-30% of the cells in ocean sediment are alive.

According to their calculations, populations of bacteria living beneath the ocean floor multiply at a rate similar to those in surface environments. The results of the study appear this week in Nature¹.

Going underground

So how could these bacteria get so deep beneath the ocean floor? "The only reasonable way is for them to be buried there," says Bo Thamdrup, a microbiologist from the University of Southern Denmark, Odense. This means that colonies of bacteria have been surviving down there for millions of years, he adds.

These microorganisms seem to produce significant amounts of methane through a metabolic process that does not rely on oxygen. Exactly how much of the global-warming gas is produced by the sub-seafloor population is not yet known. “We need future research to answer this question quantitatively,” Neretin explains.

The discovery of thriving bacteria at great depths has other, more far-reaching implications. “This study brings us closer to understanding the limits of life,” says Neretin. He adds that the cold temperatures and high pressures under which these marine bacteria survive lend support to the idea that life could exist in extreme conditions on other planets.

"It's well known that bacteria living in deep oceans have special adaptations that help them survive," says Thamdrup. "I'm sure these bacteria must have special adaptations too." A full genetic analysis could reveal their survival secrets, he adds.