They suggest the process involved may be boosted by climate change, releasing extra methane into the atmosphere and acting as a positive feedback mechanism.
Compared with the air above, most oceans’ surface waters are supersaturated with methane. The gas was thought to be produced only under anaerobic conditions, such as in sediments of paddy fields, swamps or the deep ocean. But because surface waters contain oxygen, the source of the excess methane was a mystery.
Professor David Karl of the University of Hawaii and his colleagues claim to have solved this "ocean methane paradox". During experiments, their seawater samples produced methane aerobically, despite expectations to the contrary.
They found that abundant and widespread marine bacteria metabolise methylphosphonate - a compound produced by marine plankton - when the nutrient phosphate is scarce. The process releases methane and is likely to contribute 1-4% of global emissions of the gas.
The authors suggest climate change may favour phosphate depletion and consequently boost methane production by reducing ocean circulation and phosphate cycling. The mechanism would further enhance greenhouse gas forcing.
The study also poses questions about the use of ocean fertilisation to boost carbon uptake. Californian company Planktos recently called off plans for a large-scale trial of the method (ENDS Report 398, p 29 ).
Fertilisation of the sea with iron would also be likely to deplete phosphate and enhance methane production. The extra methane would wipe out some of the already shaky gains from enhanced carbon sequestration.
This is the second methane mystery solved in recent years. In 2006, a German study discovered plants can also produced methane under aerobic conditions (ENDS Report 372, p 21 ). Although the environmental relevance of their experiments has since been questioned, it seems that tropical forests do contribute significantly to the global methane budget.