Researchers collect water samples to study bacteria and methane gas. Photo by Elizabeth Crapo/NOAA
As soon as oil began spewing into Gulf of Mexico waters, bacteria went to work, gobbling up mass amounts of methane. And as the oil continued to spill, the bacteria bloomed swiftly to consume the growing quantities of gas.
The rate at which the bacteria broke down methane in the Gulf may be the fastest ever recorded in ocean waters, said David Valentine, professor of microbial geochemistry at the University of California, Santa Barbara, and one of the lead authors of the study published online Thursday in the journal, Science.
“Whether or not it’s the fastest ever observed, it’s the fastest I’ve either observed myself or seen published,” Valentine said.
Methane is a dangerous greenhouse gas, more potent than carbon dioxide, and scientists worry about it escaping from the oceans into the atmosphere. This study indicates that the ocean can rapidly filter out methane fast enough to prevent this from happening, Valentine said.
Valentine’s team used a technique called gas chromatography to track the fate of methane released from the Deepwater Horizon disaster, when the BP well blew out in April, spilling tens of million gallons of oil into the Gulf.
Methane made up about 20 percent — some 200,000 tons — of that material. After studying natural gases released by the spill in June, Valentine’s team returned on the NOAA ship Pisces in August to track how quickly methane was degrading.
Their research before the spill had focused on methane that bubbles up naturally from the bottom of the ocean. The seafloor is a massive reservoir of methane, and the gas typically gets consumed rather slowly, said John Kessler, lead author and professor of oceanography at Texas A&M University. But the post-oil spill data told an entirely different story, one that took them by surprise.
“We anticipated that the Deepwater Horizon methane would be in the Gulf for years based on that information,” Kessler said. “When we went back in September to get a timepoint, lo and behold, we saw an endpoint.”
The team collected thousands of samples in the water and atmosphere, spanning an area of 36,000 square miles around the wellhead. They analyzed the water for methane and bacteria. By extracting DNA from trapped bacteria, they discovered that many of the microscopic critters were methane-specific, specialized to attack and degrade the stuff. But methane was absent from the water.
“It’s like the cat that ate the canary sitting by the birdcage with the feathers sticking out of its mouth,” Valentine said.
Within four months, their study concludes, bacteria had consumed nearly all of the methane released from the well’s initial blowout.
Kessler says the results provide a snapshot of how the ocean might handle large releases of methane. “This shows the capability of the natural system to respond to these types of events … and as the world’s energy demands increase, this is potentially not the last time we’ll see an event like this.”
The oil spill allowed for what Valentine calls “a forbidden experiment,” with oil and natural gases at levels that scientists could never replicate on their own. “It provided an opportunity for us to ask what happens in these large-scale methane events. And given this sort of emission, the ocean has a capacity, a very rapid capacity, to filter out that methane before it reaches the atmosphere, where it’s a potent greenhouse gas.”
However, they caution, this is only part of the story. “The methane,” Valentine said, “is not the compound of greatest concern in terms of toxicity.” Of most concern, he said, are the aromatic hydrocarbons, which exist in liquid oil and dissolve into water.
While these findings give insight into methane degradation, they tell us little about the oil remaining in the Gulf.