Scientist links sea methane to bacteria
A potential new source of methane -- a more potent greenhouse gas than carbon dioxide -- has been identified in ocean samples collected in Hawaii.
"It has to do with bacteria degrading," said David Karl, a University of Hawaii oceanographer and microbial biologist.
The discovery, which could have significant implications for global warming, was reported by Karl and his colleagues at the Center for Microbial Oceanography: Research and Education in a recent issue of the journal Nature Geoscience.
Scientists have been tracking three major greenhouse gases -- carbon dioxide, nitrous oxide and methane -- Karl explained in an interview. He said methane was believed to form only in environments with no oxygen. But scientists could not explain why oxygen-rich ocean surface waters were supersaturated with methane. The high concentrations were out of balance with methane levels in the atmosphere, he said.
Working with Karl and his team "to solve that enigma" were Edward DeLong of the Massachusetts Institute of Technology and his colleagues. Karl directs the Center for Microbial Oceanography, and DeLong is co-director.
They combined 20 years of data from observations at Station Aloha, about 60 miles north of Oahu, in the Hawaii Ocean Time-series program with new molecular tools developed by DeLong's group.
The new technology allowed the scientists to look for the presence of genes that break down certain compounds. They found their answer in methylphosphonate, an unusual organic compound just discovered in the 1960s, Karl said.
Aerobic growth of certain bacteria on the compound in the laboratory can lead to methane production, but methylphosphonate degradation in the ocean had not been considered as a source of methane production in the sea, Karl said.
He said the open ocean off Hawaii contains a high percentage of organisms that break down methylphosphonates. "Voila! You get methane."
Not only is it a potential new source of methane, but it also comes from environments containing oxygen, he said. "That's what makes it remarkable that we haven't found it before. All world ocean has oxygen, so this means this process could be going on through most of the world's ocean and most of the land as well."
Methane has contributed about 20 percent to Earth's warming since pre-industrial times, with the world's oceans contributing 1 to 4 percent of the annual global emissions, the scientists reported.
The volume of methane in the atmosphere is considerably less than carbon dioxide, but methane has a greater impact than carbon dioxide on a per-weight basis because it is better at trapping long-wavelength radiation, the researchers said. This keeps the earth habitable but also increases greenhouse warming.
People had been measuring methane in sea water and found the concentrations were higher than in the atmosphere, Karl said. "If methane was inert in the ocean, its concentration should be constant and nearly equal to the concentration in the atmosphere."
He said he spent a decade trying to find out where the methane came from. He became frustrated because he was working under the expectation that methane could be found only in environments without oxygen, he said.
In the late 1980s or early 1990s, he said, he published a paper about testing a hypothesis that methane was formed in an oxygenated environment. "I always thought maybe there was another pathway to production of methane in the open ocean. I always wanted to solve that enigma."
DeLong said the next step is "to learn how and when 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 'how' of this newly discovered methane-generating process in the sea."
Karl said the new ocean source of methane production must be incorporated into global climate thinking. "We have to know how methane from the ocean will change as climate changes."
If the ocean warms as predicted, he said, it will accelerate methane production, and that methane will go into the atmosphere and cause the greenhouse effect to accelerate, further warming the ocean.
"It will feed back on itself and go on and on until something else happens."