Deepwater Horizon methane lingered longer than thought

Much of the methane released by the 2010 Deepwater Horizon blowout hung around until the end of that year. In a new study of data collected in the months after the spill, scientists report that the numbers of methane-munching microbes plummeted while the gas was still abundant. The result contradicts an earlier report, which suggested that the bacteria rapidly got rid of the seawater’s methane.

After the Deepwater Horizon drilling rig exploded on April 20, 2010, a huge plume of pressurized oil and gas poured into the Gulf of Mexico. Scientists estimate that 80 percent of the gas was methane (SN 7/23/2011); bacteria that eat methane soon showed up for a feast. In mid-August 2010, just over a month after the gushing well was capped, researchers found methane concentrations in Gulf waters similar to amounts naturally released from the sea floor. The team concluded in 2011 that microbes had mopped up most of the spilled gas.

But others doubted that bacteria could have gobbled up all the Deepwater Horizon methane so quickly (SN 1/29/2011). To decipher what was happening, a team led by marine ecologist Samantha Joye of the University of Georgia in Athens, analyzed methane concentrations and methane consumption rates that the researchers had recorded during 10 research cruises, one conducted in March 2010, before the spill, and the rest conducted between May and December 2010.

Joye’s team reports May 11 in Nature Geoscience that methane consumption by microbes spiked by a factor of 50,000 through early June 2010. But consumption then plummeted to one-fiftieth of its maximum value by late June, even though gas was still gushing from the well. By that time, the methane plume had begun to disperse into a larger volume of water, but Joye’s team continued to find methane concentrations up to 1,000 times higher than prespill levels in some places. The scientists found that microbes were consuming their still-abundant food more slowly than expected.

Joye suspects the initial microbial bloom may have depleted the water of trace nutrients the bacteria needed to grow, or that other tiny ocean dwellers ate or infected the bacteria. Her team’s study highlights how little scientists know about how microbes in the ocean react to gas influxes, she says. “You cannot simply assume they’re going to consume all the methane.”

Understanding methane-eating microbes’ behavior is critical, marine geochemist Evan Solomon of the University of Washington, in Seattle, writes in a commentary accompanying the new study in Nature Geoscience. Deep-sea oil exploration will probably release future spills, Solomon says, and global warming could destabilize large undersea deposits of frozen methane, leading to local ocean acidification or oxygen depletion (SN 7/31/2010). He calls the study “a good step forward” for scientists’ understanding of what happens to undersea methane. The gas could substantially increase greenhouse warming if released from the ocean in large amounts.

University of California, Santa Barbara geochemist David Valentine, a researcher involved with the 2011 study, is skeptical that the methane munchers died off before gobbling up the gas. He says he has “pretty serious concerns” that the researchers sometimes went to their lab to measure how fast the bacteria consumed methane. The collected microbes might have greatly expanded in number before the measurements, Valentine says, creating artifacts in the study’s bacterial numbers and methane consumption rates.

Joye says her team accounted for that possibility by measuring how fast methane-munchers’ populations grow: not very. This means the team’s lab measurements accurately reflect what was happening in the Gulf even several weeks earlier, Joye argues.