These are boom times for oil-eating microbes in the deep waters of the Gulf of Mexico, thanks to BP's Deepwater Horizon accident that has added some 600 million liters of hydrocarbons to those waters. And now research published online in Science on August 24 shows that an array of new and unclassified oil-eating bacteria are feasting on the newly rich resource of hydrocarbons.
Microbial ecologist Terry Hazen of Lawrence Berkeley National Laboratory (LBNL) and his colleagues used two ships to collect 200 samples from 17 deep water locations between May 25 and June 2. The samples revealed a broad array of microbes, closely related to those of the Oceanospirillales family that includes such oil eaters as Oleispirea antarctica, Oceaniserpentilla haliotis and Thalassolituus oleivorans. These new microbes—led by one particular unusually long (20 microns), yet to be named species that made up more roughly 95 percent of the community responding to the spill—had consumed roughly 8 percent of the available oxygen in these cold, deep waters as they busily converted hydrocarbons into more microbial cells, limited only by a lack of iron. As a result, oxygen saturation at depth dropped from an average of 67 percent to 59 percent while microbial cell concentrations doubled to 5,510 cells per milliliter compared with just 2,730 cells per milliliter outside the plume. So far, no one has measured an impact on other ocean life from the drop in oxygen, which is unexpectedly small, a fact that Hazen ascribes to the dispersed nature of the oil plume. "Concentrations of oil in this deep plume are so low that we don't see oxygen depletion," he explains.
The microbes did this at extremely low temperatures—4.7 degrees Celsius—that slow ordinary bacterium's metabolism as well as under high pressures. And it was a group of roughly 16 gamma-Proteobacteria species doing the bulk of the work. Genetic testing revealed these microbes engaged the same cellular equipment as known oil-eaters, such as Alcanivorax borkumensis from the Oceanospirillales order, to break down the hydrocarbons. Large quantities of the various genes necessary to such oil-eating work were found in hydrocarbon-contaminated waters compared to untainted waters.
The scientists also found that the microbes were making quick work of many of the hydrocarbons in the deep sea plumes. Alkanes, for instance—a hydrocarbon easily digested by such bacteria—could be consumed in as little as a day. In fact, the alkanes in these samples lasted, at best, roughly six days in the presence of microbes. Hazen and his colleagues attribute that quick work to the large contingent of such volatile hydrocarbons that are readily biodegradeable in the Louisiana light sweet crude spilled as well as the breaking up of the oil into small particles by chemical dispersants.
"Microbes eat [hydrocarbons] like we eat at a buffet: they like some compounds better than others," explained marine chemist Chris Reddy of the Woods Hole Oceanographic Institution during an August 19 press conference announcing the existence of such oil plumes—though their findings also published in Science suggested microbes were consuming the plume slowly in late June due to low levels of oxygen depletion. "They leave behind a fingerprint of what they chose to eat," Reddy explained.
In fact, some scientists disagree with the analysis of the biodegradation rates by Hazen and his colleagues. "The microbiology is very convincing," but the extrapolation to the rate of hydrocarbon consumption "is potentially flawed and gives the wrong impression," says biogeochemist David Valentine of the University of California, Santa Barbara, who is also attempting to characterize the microbial response to the oil spewing from BP's Macondo 252 well in the Gulf of Mexico. "They are probably measuring the dilution time for the early plume, more so than the biodegradation rate." In other words, the microbes don't eat the alkanes in the hydrocarbon plume in less than a week; rather these compounds get diluted among all the trillions of gallons of seawater in the Gulf.