Dispersants affect the evolution of oil-degrading microbial populations. (A) Average and standard deviation (SD) of cell numbers from sample triplicates (log scale) monitored for 6 wk in microcosms. (B) Relative abundance of bacterial groups in Gulf of Mexico deep water in situ samples and in the microcosms (average of triplicate samples). Reads of the V4V5 regions of the 16S rRNA gene were clustered into operational taxonomic units and taxonomy was assigned with Global Alignment for Sequence Taxonomy (GAST). Graphic: Kleindienst, et al., 2015 / PNAS

By Seth Borenstein
9 November 2015

(PhysOrg) – The chemical sprayed on the 2010 BP oil spill may not have helped crucial petroleum-munching microbes get rid of the slick, a new study suggests.

And that leads to more questions about where much of the Deepwater Horizon oil spill went. If the new results are true, up to half the oil can't be accounted for, said the author of a new study on the spill in the Gulf of Mexico.

After the 172 million gallon (650 million liter) spill, the chemical dispersant Corexit 9500 was applied by airplane on the slick to help it go away and help natural microbes in the water eat the oil faster. The oil appeared to dissipate, but scientists and government officials didn't really monitor the microbes and chemicals, said University of Georgia marine scientist Samantha Joye.

So Joye and colleagues recreated the application in a lab, with the dispersant, BP oil and water from the gulf, and found that it didn't help the microbes at all and even hurt one key oil-munching bug, according to a study published Monday in the journal Proceedings of the National Academy of Sciences.

"The dispersants did a great job in that they got the oil off the surface," Joye said. "What you see is the dispersants didn't ramp up biodegradation."

In fact, she found the oil with no dispersant "degraded a heckuva lot faster than the oil with dispersants," Joye said.

Joye's team chronicled nearly 50,000 species of bacteria in the Gulf and what they did to the water with oil, and water with oil and dispersant.

One of the main groups of oil munchers are fat little sausage-shaped bacteria called marinobacters, Joye said. They eat oil all the time and comprise about 3 percent of the bacteria in normal water. But when there's oil, they eat and multiply like crazy until they are as much as 42 percent of the bacteria, Joye said.

But when the dispersant was applied, they didn't grow. They stayed around 3 percent, Joye said.

Instead, a different family of bugs called colwellia multiplied more, and they don't do nearly as good a job at munching the oil, Joye said. She theorized that for some reason the dispersant and marinobacters just don't work together. [more]

Study: Dispersants did not help oil degrade in BP spill

ABSTRACT: During the Deepwater Horizon oil well blowout in the Gulf of Mexico, the application of 7 million liters of chemical dispersants aimed to stimulate microbial crude oil degradation by increasing the bioavailability of oil compounds. However, the effects of dispersants on oil biodegradation rates are debated. In laboratory experiments, we simulated environmental conditions comparable to the hydrocarbon-rich, 1,100 m deep plume that formed during the Deepwater Horizon discharge. The presence of dispersant significantly altered the microbial community composition through selection for potential dispersant-degrading Colwellia, which also bloomed in situ in Gulf deep waters during the discharge. In contrast, oil addition to deepwater samples in the absence of dispersant stimulated growth of natural hydrocarbon-degrading Marinobacter. In these deepwater microcosm experiments, dispersants did not enhance heterotrophic microbial activity or hydrocarbon oxidation rates. An experiment with surface seawater from an anthropogenically derived oil slick corroborated the deepwater microcosm results as inhibition of hydrocarbon turnover was observed in the presence of dispersants, suggesting that the microcosm findings are broadly applicable across marine habitats. Extrapolating this comprehensive dataset to real world scenarios questions whether dispersants stimulate microbial oil degradation in deep ocean waters and instead highlights that dispersants can exert a negative effect on microbial hydrocarbon degradation rates.

Chemical dispersants can suppress the activity of natural oil-degrading microorganisms



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