Though it sounds like a swank new aftershave or maybe a rap star, "methane ice" refers to the vast deposits of frozen methane buried in the sea floor. Marine scientists have wondered why so little of this methane seeps from the ice into the ocean. Now they think they've solved the mystery: Microbes in the sediments slurp it up first. The finding might help explain the huge amounts of methane that occasionally belch from the seafloor, an event linked to rapid climate change.
Along the margins of the continents, crushing pressures and near-freezing temperatures squeeze methane and water together in the sediments to form methane ice, or methane hydrates. Because little dissolved methane shows up in the overlying ocean, scientists used to surmise that microbes inhabiting the ocean floor were sopping up any methane that escaped from the ice and transforming it into carbon dioxide. But in 3 decades of searching, no one had captured any likely microbe suspects.
Now Antje Boetius of the Max Planck Institute for Marine Microbiology in Bremen, Germany, and colleagues have found two such suspects in sediment cores collected off the coast of Oregon. They treated the samples with fluorescent probes, molecular tags designed to bind to a particular nucleotide sequence in microbial RNA. The researchers report in the 5 October issue of Nature that they saw glowing, double-layered nodules of cells. Clustered at the core of nodules were methane-consuming microbes known as archaea, while a thin layer of sulphate-using bacteria coated the spheres. Though the scientists didn't work out the exact chemistry, they think the archaea cleave methane molecules and then pass some of the byproducts to the bacteria, which use them as an energy source.
Methane breakdown is a globally important process because "it keeps massive quantities of methane--a potent greenhouse gas--from reaching the overlying waters and atmosphere," says microbiologist David Valentine of the Scripps Institution of Oceanography in La Jolla, California. Understanding how the methane concentration of the ocean floor is regulated, he adds, may give scientists insights into periodic methane "blowoffs" that seem to accompany periods of global warming.