Researchers Identify New Pathway for Methane Production in the Oceans

3 July 2008

Researchers from the University of Hawai’i at Mânoa and MIT have identified a new pathway for methane production in the oceans, suggesting that aerobic decomposition of methylphosphonate, an organic, phosphorus-containing compound, may be responsible for the supersaturation of methane in ocean surface waters. A paper on the work is published in the journal Nature Geoscience.

Today, between 20-30% of the total radiative forcing of the atmosphere is due to methane, a potent greenhouse gas with a global warming potential of 21, compared to CO₂’s GWP of 1. Terrestrial sources of methane production are well known and studied (including extraction from natural gas deposits and fermentation of organic matter), but those known sources did not account for the levels of methane observed in the atmosphere.

In investigating this “methane enigma”, lead author David Karl of the University of Hawai’i found a possible explanation in the compound methylphosphonate, an unusual organic compound only discovered in the 1960s. In the laboratory, the aerobic growth of certain bacteria on methylphosphonate can lead to the production of methane, but until now this process of methylphosphonate degradation in the ocean had not been suggested as a possible pathway for the aerobic production of methane in the sea.

When people began measuring methane in the ocean, they found that methane concentrations varied with geographical location and with water depth. If methane was inert in the ocean, its concentration should be constant and nearly equal to the concentration in the atmosphere. What the scientists found was that methane was lower than expected in deep waters, implying net consumption by microbes. However the big surprise was that near surface concentrations were higher than in the overlying atmosphere which indicated a local production of methane in the sea. Because methane is produced only in regions devoid of oxygen and since the surface ocean contains high oxygen levels this was very perplexing.

—David Karl

Although the implications for global climate change are still being studied, the warming and further stratification of the ocean seem likely to affect marine methane production.

This is a newly recognized pathway of methane formation that needs to be incorporated into our thinking of global climate. Since our oceans cover ¾ of the planet, you just need to stimulate this pathway a little bit and you’re going to create more methane. And one way you can tweak it is to stratify the oceans, which we know will happen. All of the climate models show that the ocean will become more nutrient limited over time.

—David Karl

The researchers will next look at how and when the  microbial communities turn on and off their methane production genes in response to the methane precursors like methylphosphonate in their natural environment.

This should provide new insights about the ‘who’ and the ‘how’ of this newly discovered methane generating process in the sea.

—Edward DeLong, MIT, co-author

This research was supported by the Gordon and Betty Moore Foundation and the National Science Foundation.

Resources

• David M. Karl, Lucas Beversdorf, Karin M. Björkman, Matthew J. Church, Asuncion Martinez & Edward F. Delong (2008) “Aerobic production of methane in the sea”, Nature Geoscience, doi: 10.1038/ngeo234