Jars of plankton taken from near the Gulf oil spill show how the water's oxygen—and thus sea life—decreased with time, a sign of increased bacteria.
Photograph by David Liittschwager, National Geographic
Surprisingly, practically all of the methane that accompanied nearly five million barrels of oil leaked into the northern Gulf of Mexico (map) has been devoured by giant bacterial blooms, a new study says. Methane, the main component of natural gas, is typically found with oil.
(Read more about how nature is fighting back against the oil spill.)
The Deepwater Horizon oil rig burned and sank last April, and the damaged wellhead on the seafloor below was permanently capped in July.
"We thought it would probably be something on the order of a year or so until the methane disappeared," said study leader David Valentine, a microbial geochemist at the University of California, Santa Barbara. "Instead we found that by mid-September it was completely gone."
Deep, water-dissolved methane was the single most abundant hydrocarbon released during the spill, making up about 20 percent of the flow from the wellhead, Valentine and his team estimated.
However, the discovery doesn't mean that the oil itself is gone. The team found oil nearly everywhere in the deep water—suggesting that oil-water mixing was widespread, and that some ingredients of the oil haven't completely degraded.
(See "Why the Gulf Oil Spill Isn't Going Away.")
Ocean vs. Oil
The Gulf spill's methane release may rival the amount of methane regularly released via deep-ocean hydrothermal vents, oil seeps, and the breakdown of methane-rich gas hydrates in ocean sediments.
(Read more about the Gulf of Mexico's natural seeps.)
Scientists are keeping a close eye on all of Earth's methane sources. That's because warming ocean temperatures are thought to make some parts of the ocean's geology less stable. This could trigger the release of huge amounts of the gas into the ocean and potentially the atmosphere, where it may contribute to global warming.
"The global ocean seafloor contains the largest reservoir of methane, a potent greenhouse gas, and there is evidence that releases of methane from this reservoir have modulated climate in the history of the planet," said study co-author John Kessler, a chemical oceanographer at Texas A&M University.
"Without these bacteria, methane could be released to the atmosphere—and if the release is large enough, it could influence global temperatures."
(Related: "Methane-Munching Microbes Take a Bite Out of Warming.")
But the oil spill—essentially an unprecedented and accidental experiment—offers a piece of good news: The bacterial rapid-response system might be able to significantly curb the effects of even massive natural methane emissions.
"What we found is that for a deep-ocean methane release, the ocean has a capacity to deal with that in pretty rapid order," study leader Valentine said.
Low-Profile Bacterial Bloom
The team studied bacteria concentrations in deep, oil-fouled Gulf of Mexico waters during the spill event and again after the wellhead was sealed.
They found a larger-than-expected presence of methane-eating bacteria, as well as low oxygen levels. Lower oxygen suggests microbial activity because the organisms breathe the gas while they devour methane.
Methane-eating bacteria—discovered only about a decade ago—are distinct from their oil-degrading relatives, which themselves went to work on other oil ingredients.
The newly discovered bacterial bloom isn't as visibly obvious as the vibrantly colored algal blooms often seen in shallower waters, Valentine noted.
"I suspect you wouldn't notice much to look at it, but there was a big change in what happened to the biological community," according to Valentine, whose study was published online January 6 in the journal Science Express.
Bacteria No Guarantee Against Oil
Antje Boetius, a microbiologist at the Max Planck Institute for Marine Microbiology in Bremen, Germany, was impressed with the study.
"We have worked at huge natural methane seeps before, and never found a situation like this with a huge propagation of bacteria in which the methane goes away so quickly—something special [such as warm temperatures] in that event really induced the bacteria to grow tremendously," she said.
But Boetius stresses, as do the study's authors, that caution is needed before assuming these findings can be applied in other places where oil spills may occur.
(See "Exxon Valdez Anniversary: 20 Years Later, Oil Remains.")
"What we've learned from oil spills in general is that the speed with which nature heals itself is very different for different accidents for a number of reasons, including water temperatures," she said.
For example, "bacteria are slowed in colder waters, and the Gulf is a rather warm place compared to locations like the polar regions—which are exactly where we'd expect global warming to have the largest impacts on huge amounts of buried methane.
"Can we deduce from these data where we have an unexpected explosion of bacteria that the same could happen in other very sensitive areas which are much colder? That’s the big question."
Gulf Bacteria Primed For Next Spill?
Now that the spill's methane is mostly cleaned up, the fate of methane-munching bacteria is another intriguing question the team will be watching closely.
"We expect that the [high amount of bacteria] will gradually dissipate, but we don’t really know how the population relaxes back to a baseline level," Valentine said.
Indeed he believes the baseline may be forever changed—and Gulf bacteria could become even more efficient in dealing with future oil spills.
(Also see "3 Future Oil-Spill Fighters: Sponges, Superbugs, and Herders.")
"Sometimes where there is petroleum contamination and a bacterial response, the environment harbors a memory of it. ... It maintains a kind of priming, so that if you contaminate it again, it will react more quickly than it would in a pristine environment."
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