Bacteria devoured methane gas from gulf oil spill, scientists say
Thursday, January 6, 2011; 3:19 PM
Last August, just two days into a research cruise to study methane gas spewed into the Gulf of Mexico by the Deepwater Horizon gusher, Texas A&M University oceanographer John Kessler turned to one of his colleagues and said, "Well, it looks like it might be gone. What do you think?"
The huge wallop of methane burped up from deep inside the earth was, in fact, missing.
Kessler and his colleagues now report in Science that a huge swarm of gas-gobbling bacteria swelled to consume nearly all of the estimated 200,000 tons of methane dumped into the gulf. Methane is the primary component of natural gas and is typically found packed together with oil and other hydrocarbons at drill sites. Before and during the spill, BP estimated that natural gas made up about 30 percent of the output of the Macondo well drilled by the doomed Deepwater Horizon rig.
Besides providing some good news for the gulf region, the finding has potential implications for climate change science, too. Methane is a much more potent greenhouse gas than carbon dioxide, and, as the earth warms, climate scientists worry that much more methane will be released from the oceans. "What this tells us is that natural releases of methane from the seafloor with similar characteristics will not make it up to the atmosphere, will not influence climate," Kessler says.
While the gusher was still flowing, in June, a team sponsored by the National Science Foundation and the National Oceanic and Atmospheric Administration had found intense concentrations of methane in the waters near the wellhead, says David Valentine of the University of California at Santa Barbara, who helped lead that work. At the time, that team detected very few of the methane-eating bacteria that naturally live in the gulf. So Valentine and colleagues arrived at a grim - though early - conclusion. "Originally, we had expected that the methane would be consumed gradually," Valentine says. "We really thought it would be around for a year or more."
Instead, two to three months after engineers finally capped the well, the gas was gone. All of the evidence points to an explosion of methane-eating bacteria.
From mid-August through early October, a team aboard the NOAA vessel Pisces searched an area of the gulf larger than Indiana, dipping a package of sensors into the water at 207 locations. They found almost no methane. Instead, their instruments recorded huge pockets of depleted oxygen, which the microbes use to burn the gas.
"If the methane had just traveled someplace else and was hiding, we wouldn't see any reductions in dissolved oxygen," Kessler says. "But if it were consumed by microbes, we should see some reductions in dissolved oxygen, which we did."
In fact, Kessler calculated that the amount of missing oxygen almost exactly equaled the amount required by a vast cloud of microbes to consume the entire bolus of methane. "The math worked out scary good," he says.
The microbes, known as methanotrophs, were discovered about 100 years ago, and they exist everywhere in the ocean scientists have searched for them. They make a living much the same way a gas fireplace provides warmth: by burning methane and oxygen to release energy, carbon dioxide and water.
Back in his laboratory, Valentine, who was also aboard the Pisces, documented the last piece of evidence: striking genetic fragments of a robust community of methanotrophs. He also discovered DNA sequences for an enzyme the microbes deploy to break methane's chemical bonds.
"In September and October we found significant numbers of methane consumers," Valentine says. "But because there was no methane for them to eat anymore, we interpret that as being the residual population that had consumed all the methane."
Tori Hoehler, who studies methanotrophs at NASA Ames Research Center in California, said the team did a good job of ruling out the possibility that the methane had simply burbled into the atmosphere. "They didn't see any methane coming out of the water column" in June, he says. "The surprise is to see that the methanotrophs can respond in relatively rapid fashion."
By contrast, Terry Hazen, a microbial ecologist at Lawrence Berkeley National Laboratory in California, says he is "not surprised at all" by the voracious appetite of the microbes. Hazen spent four months on the gulf studying the giant plumes of oil and gas that initially spread from the well. "We have basically the same data," he says. In August, Hazen reported that bacteria - including several new-to-science species - were quickly reducing the giant plumes of oil. "The good news is that methanotrophs will probably help to deep clean" the gulf by continuing to degrade residual oil and gas, Hazen says.
However, Hoehler says the findings offer a double-edged lesson. "There is a built-in capacity for the ecosystem" to deal with oil and gas spills, he says. "That is in some ways a comforting thought. But if people simply have the notion that bacteria will clean up our messes for us, then there will be less of a concern about making messes in the first place. It's nice to know that [bacteria] are there to fix what we've done wrong. But it's not a crutch you want to lean on."