Recirculation aided Gulf plume’s degradation

Research offers new explanation for disappearance of subsea oil and gas

Throughout the months-long 2010 BP oil spill in the Gulf of Mexico, scientists expressed surprise at the development and relatively speedy disappearance of giant plumes of subsea oil and gas that jetted from the wellhead and refused to surface. A new study explains how bacteria degraded the plumes so efficiently: A succession of hydrocarbon-noshing species mushroomed because their movable feasts were repeatedly replenished.

Only about 15 percent of the BP gusher floated up to form giant surface slicks, a second new study finds. Natural gas constituents and dissolvable chemicals amounting to twice that mass remained near the seafloor, creating the roving cloudlike hydrocarbon plumes on which the bacteria fed.

Besides natural gases, the plumes contained all of several highly volatile crude oil constituents released in the spill, including benzene and toluene. So cleanup workers who say they were sickened by benzene in the surface slick, as evidenced by elevated blood levels of the carcinogen, may have been exposed from other sources, concludes the leader of the second study, Thomas Ryerson of the National Oceanic and Atmospheric Administration in Boulder, Colo.

Both studies are being published online the week of January 9 in the Proceedings of the National Academy of Sciences.

For their plume-degradation study, David Valentine of the University of California, Santa Barbara, and his colleagues adapted a Navy computer program that predicted Gulf currents and incorporated information newly gleaned from the Gulf and from lab studies. The researchers established which hydrocarbons were present, where plumes had been recorded and which Gulf bacteria had a propensity for eating plume compounds.

Then the researchers used the computer program to predict the bugs’ feeding rates — and plume degradation — based on established periods of cell division that varied by species and food availability. Although bacteria should have depleted the available oxygen as they ate and reproduced, the computer program showed there would have been substantial water mixing, replenishing oxygen supplies.

Most parcels of the oiled water in which the bacteria were riding circled back to the seafloor wellhead multiple times, the Navy’s current data indicated. This would have restocked the bacteria’s hydrocarbon smorgasbord and further fueled mushrooming populations of these bugs.

“What happened in the deep plumes was far more complex than any of the initial papers had really acknowledged,” says chemical oceanographer Benjamin Van Mooy of the Woods Hole Oceanographic Institution in Massachusetts. Valentine’s team, he says, “offers a powerful way of distilling that complexity and begins to tie up some loose ends.” He notes his own team had trouble explaining why oil showed up — or didn’t — during their cruises amid the spill. “That had been a sort of head-scratcher,” says Van Mooy, adding that the Valentine paper reassures him “that I wasn’t going totally insane out there.”

Although the role of water mixing in sustaining biodegradation is almost obvious, says Annalisa Bracco of the Georgia Institute of Technology, it’s never been considered in the framework of bacterial growth from the spill. Mixing’s role in the new paper, she says, “could indeed help to solve the problem of explaining very high levels of bacterial degradation required to consume all or most of the gas and oil from the [BP] spill.”

Bracco and Van Mooy emphasize that the new findings rest on computer analyses that should be confirmed against real-world observations or further lab testing.

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

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