By Janet Raloff, Science News
Methane, the predominant hydrocarbon produced by the BP blowout last year, has all but vanished from Gulf of Mexico waters, a new study reports—presumably eaten up by marine bacteria. That hadn’t been expected to happen for years.
Two-thirds of the hydrocarbons released by the BP accident were forms of natural gas: largely methane, ethane and propane. While Gulf microbes quickly began devouring the larger gas molecules, they initially left tiny methane—which accounted for an estimated 87.5 percent of the gas initially emitted—largely untouched.
Some of the authors of the new paper had reported in the Oct. 8 Science finding almost no microbial breakdown of BP methane in June, about a month and a half into the 83-day gusher.
Rates of biodegradation in subsea plumes, where this gas had been accumulating, “indicated methane would persist for many, many years, if not almost a decade,” observes John Kessler, a chemical oceanographer at Texas A&M University in College Station and an author of that earlier report.
To begin quantifying just how slowly that breakdown was proceeding, he and his colleagues returned to the Gulf for three research cruises between August 18 and October 4. Their sampling at more than 200 sites turned up no BP methane. In fact, concentrations of the gas in seawater throughout the spill zone were lower than typical background concentrations for the Gulf, these researchers report online January 6 in Science.
“We were caught off guard,” Kessler says. “But that highlights the beauty of the scientific process. You put together hypotheses based on the information at hand and test them. And whether we’re right or wrong, at the end of the day we’ll have learned something new about the system.”
Not so fast, counters another BP-plume investigator, marine ecologist Samantha Joye of the University of Georgia in Athens. “Just because you can’t find methane in the spot where you lowered your [instruments] doesn’t mean there’s no methane out there somewhere,” she says.
Her team and others tracked plumes migrating out from the wellhead early in the spill. Deep and persistent, these diffuse clouds of oil and gas proved very localized and mobile, she says. From late June until early August, however, no one continued to track their whereabouts, Joye observes. So “the more parsimonious explanation” for why Kessler’s group found no BP methane: “ They lost track of the freaking plume.”
David Valentine of the University of California, Santa Barbara, a coauthor of the new report, disagrees. “The ocean doesn’t flow like a river,” he says. “There’s a lot of mixing, flowing backward and turning around in circular motions. So the water at the wellhead still contains some of the water that’s been there over the last couple months.” And in this water and moving southwest for “a couple hundred miles,” he says his team “saw evidence of microbial activity—throughout the whole distance.”
He points to a huge drawdown in dissolved oxygen, which methane-munching bugs—called methanotrophs—would have consumed. “We saw a loss of about a million tons of dissolved oxygen,” he says, which would explain the disappearance “of almost all of the methane, ethane and propane and some of the hydrocarbons in oil.”
Moreover, his group saw almost no methanotrophs in plume zones last June, but plenty of these and other oil-degrading species in subsequent voyages.
The new paper’s conclusions “are quite consistent with what we’ve seen,” says microbial ecologist Terry Hazen of Lawrence Berkeley National Laboratory in California. On August 24, his team was the first to report online in Science that BP oil plumes had disappeared.
The new paper’s report of an evolution in the types of microbes present in the plume residues “is dramatic and very convincing,” says oceanographer Ian MacDonald of Florida State University in Tallahassee. But, he adds, the paper contains too little data to convince him that microbes really ate up all of the methane. It’s possible, he says, that what the researchers sampled was merely the wake of a passing plume and its bacterial stragglers.