Marine Methane Mystery Explained

AEROBIC BREAKDOWN of methylphosphonate into methane by marine microbes may account for high levels of methane found in ocean surface waters, according to a new report in Nature Geoscience (2008, 1, 473).

Methane is a potent greenhouse gas, and oceanic contributions to the global methane budget are not well understood. "Why methane concentrations in the oxygen-rich surface waters of most of the world's oceans are so high has remained a puzzle, as methane production is thought to occur in strictly anaerobic environments," writes Ellery D. Ingall, an associate professor at Georgia Tech who studies marine phosphorus cycling, in a commentary accompanying the report.

Living organisms need phosphorus to grow. Phosphonates abound in the ocean but make up only a small percentage of total marine phosphorus nutrients. Although most microorganisms prefer to munch on more common organic phosphorus compounds, it's likely that they adapt to phosphonates in nutrient-poor regions of the ocean and generate methane, Ingall notes.

Previous laboratory studies had shown that aerobic bacteria feeding on methylphosphonate produce methane, but that pathway had not been proposed to account for the methane in oceanic surface water. To test that idea, University of Hawaii oceanographer David M. Karl and colleagues there and at MIT collected samples of oxygenated seawater off the coast of Hawaii. In the lab, they spiked methylphosphonate into samples containing native microorganisms. After incubation, they found methane in the samples, indicating that the microbes aerobically decompose methylphosphonate to methane. Karl says his group is now collecting methane data from field experiments to confirm the laboratory experiments.

If bacteria metabolize even a small fraction of methylphosphonate in the ocean, they would produce enough methane to keep marine surface waters saturated with the gas, the researchers say. In addition, they believe that marine methane production may increase in response to climate change, as oceans warm and the availability of nutrients decreases.