Methane seeps, which play a role in the global dynamics of this potent greenhouse gas, may be even more complicated than we thought. A perspective in this week's Science describes several new research papers on the release of methane at the level of the ocean floor. Their conclusions run counter to those derived from work three decades old, and suggest that many bacteria thought to be consuming the ocean's methane may actually be producing it.
Many studies in the past indicated that methane from ocean sediment was gobbled up by anaerobic bacteria before it even reached the ocean floor. However, new studies of methane seeps, where methane bubbles up from the ocean floor in large quantities, seem to suggest that there are more complicated mechanisms at work.
The authors compare studies done on two different marine environments: one off the coast of Alaska, where methane leaks slowly from beneath the ocean floor, and another off the coast of Oregon, where methane bubbles up in large quantities. It stands to reason that the Oregon scenario would have a higher population of methane-consuming bacteria turning it into carbon dioxide—more food, more consumers. But this was not the case.
The research in Oregon showed that 100 times as much methane escaped as in Alaska. But it wasn't just because the bacteria were lazy: the reaction rates at the level of the sediment showed an energy flux in Oregon 3,000 times that in Alaska. The bacteria were somehow managing to do more work despite processing less methane.
The scientists postulate that, in heavy methane seep scenarios, some of the methane-consuming bacteria may run "in reverse," creating methane from ambient hydrogen and carbon dioxide. The bacteria may be motivated to do this because of the high flux of hydrogen molecules created by their forward-running brethren, which are consuming and processing methane.
The ocean's methane seeps generate the equivalent of half that produced by all natural land sources, though only a tiny portion escapes to the surface. The authors note that we won't be able to understand this small contribution without some teamwork between the fields of microbiology and geochemistry.