Two microbes team up to munch methane.

Vast stores of methane gas lie buried beneath the seafloor, yet little escapes from the sediments into the ocean and the atmosphere above. Geochemists have long suspected that methane-munching microbes gobble the gas before it can seep upward out of the ooze. But scientists had been at a loss to explain how a microorganism could consume methane where oxygen is in short supply, as it is in the sediments.

As it turns out, not one microbe but two probably work as a tag team to feed off the methane. A group of researchers studying methane-bearing sediments collected off the coast of Oregon has discovered aggregates of two fundamentally different microorganisms, which appear to be collaborating to consume the gas. Led by Antje Boetius, a microbiologist at the Max Planck Institute for Marine Microbiology in Bremen, Germany, the scientists report their findings in the Oct. 5 NATURE.

Most methane in Earth's atmosphere is produced in low-oxygen environments by a class of single-celled microbes known as archaea. However, in the presence of bacteria that use sulfate ions rather than oxygen to fuel their metabolism, some species of archaea can consume methane rather than produce it.

Boetius' team found cell aggregates that typically had a core of about 100 methane-consuming archaea. This dense sphere of cells was fully or partially surrounded by a shell of about 200 sulfate-consuming bacteria. These cellular consortia measured on average, 3.2 micrometers across.

The highest concentration of the cellular clusters--more than 30 million per cubic centimeter--occurred within the top 5 cm of sediment. There, the concentration of sulfate ions from the ocean water and the rate at which the sulfate ions are consumed were also highest.

Edward F. DeLong, a microbiologist at the Monterey Bay Aquarium Research Institute in Moss Landing, Calif., says it's likely that the bacteria in the outer shell use the sulfate ions in the shallow sediments to help them metabolize the organic compounds produced by the methane-consuming archaea.

DeLong says the tight clustering of the two microorganisms is "a solid piece of evidence" that the groups of cells have formed an intimate partnership enabling them to feed off the methane.