How Microbes Teamed to Clean Gulf

By GAUTAM NAIK

Associated PressA plume of oil is shown in May 2010 in the waters of Chandeleur Sound, La., in the wake of the explosion of the Deepwater Horizon offshore oil platform the previous month.

A fortuitous combination of ravenous bacteria, ocean currents and local topography helped to rapidly purge the Gulf of Mexico of much of the oil and gas released in the Deepwater Horizon disaster of 2010, researchers reported on Monday.

After spewing oil and gas for nearly three months, the BP PLC well was finally capped in mid-July 2010. Some 200,000 tons of methane gas and about 4.4 million barrels of petroleum spilled into the ocean. Given the enormity of the spill, many scientists predicted that a significant amount of the resulting chemical pollutants would likely persist in the region’s waterways for years.

According to a new federally funded study published Monday by the National Academy of Sciences, those scientists were wrong. By the end of September 2010, the vast underwater plume of methane, plus other gases, had all but disappeared. By the end of October, a significant amount of the underwater offshore oil—a complex substance made from thousands of compounds—had vanished as well.

“There was a lot of doomsday talk,” said microbiologist David Valentine of the University of California, Santa Barbara, and co-author of the study, published in Proceedings of the National Academy of Sciences. But it turns out “the ocean harbors organisms that can handle a certain amount of input” in the form of oil and gas pollutants, he said.

A year ago, Dr. Valentine and other scientists published a paper describing how bacteria that feed on naturally occurring oil and gas leaks underwater had apparently devoured much of the toxic chemicals released in the BP spill. That federally funded study, published in the journal Science, triggered disbelief among other researchers who questioned whether microbes could gobble up that much gas and oil so quickly.

Dr. Valentine and colleagues have now used a computer model to explain just how that scenario might have played out, though some scientists remain skeptical.

It was an intricate challenge. The first step was to estimate the flow rate of the various hydrocarbons from the well over the 87 days that the spill continued. The researchers identified 26 classes of such chemicals; they then had to figure out which of these chemicals stayed in the deep plume that remained more than 3,000 feet underwater, and which ones rose up to the surface. For example, in the plume, certain chemicals dissolved completely in the water, including the methane gas, while some of the oil droplets were atomized and remained suspended in the water. A lot of the surface oil evaporated or washed up on Gulf shorelines.

Next, the scientists set about identifying the main species of oil-and-gas-eating bacteria that lived in the deep Gulf. They identified 52 main species of such microbes. The scientists also estimated how quickly the bacteria consumed oil and gas and how much the bacteria colonies grew.

The final step was to model the complex movement of the water in the Gulf to determine where the oil and gas—and the bacteria—got transported. Igor Mezic, a colleague of Dr. Valentine’s and also a co-author, had published a study in 2011 predicting where the BP oil slick had spread. That analysis included data from the U.S. Navy’s model of the Gulf’s ocean currents and observations of the water’s movements immediately after the spill and for several months after it ended.

The UC Santa Barbara researchers decided to marry their two computer models—the one about the spill-eating bacteria with the one capturing the movement of water. When they ran the joint model, they found that it helped to explain the puzzle of the rapidly vanishing oil spill.

The model showed that the topography in the Gulf had played a vital role. Because the Gulf is bounded on three sides by land—north, east and west—the water currents don’t flow in a single direction as in a river. Instead, the water sloshes around, back and forth, as if it were trapped in a washing machine.

An initial population of bacteria encountered the spill near the BP well, its population grew, and then it was swept away by the ocean currents. But when the water circled back—that washing-machine effect—it was already loaded with these hungry bacteria, which immediately went on the attack again, mopping up another round of hydrocarbons. These repeated forays over the BP well, by the ever-growing bacterial populations, sped up the rate at which the methane and offshore oil got devoured.

Dr. Valentine suggested that oil companies ought to ascertain the currents, water motion and native microbial community in the water before embarking on any major offshore drilling project. “Then, if there is an event, we’d be many steps ahead of understanding where the oil may go and what the environment’s response may be,” he said.

Ira Leifer, a petroleum geochemist also at UC Santa Barbara who co-wrote a rebuttal to the 2011 paper published in Science, said the latest study was limited because it was based on a computer model “which is only as good as the input or assumptions” on which it is based. He noted, for example, that the authors had neglected to include a discussion of whether the bacteria would run out of critical nutrients necessary for them to consume the oil and gas and reproduce.

The research was funded by the National Science Foundation, the Department of Energy and the Office of Naval Research.

Write to Gautam Naik at gautam.naik@wsj.com