Marine microbes offer a plausible explanation for the surprising abundance of methane in oxygenated parts of the ocean.

Scientists have previously theorized that ocean methane might be a by-product of microorganisms' use of methylphosphonic acid as a source of phosphorus. But it was unclear where the acid itself came from. William Metcalf and Wilfred van der Donk at the University of Illinois in Urbana and their colleagues show that a microbe called Nitrosopumilus maritimus carries genes that encode a pathway for methylphosphonate synthesis.

A crucial gene in this pathway is also found in many other marine microbes, suggesting that these organisms may be the source of the unexplained ocean methane.

Science 337, 1104–1107 (2012)