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Vol. 343 ,
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Toxicity Aside, Dispersants Could Undermine Natural Oil-Eaters
26 May 2010 3:07 pm
Dispersants, which include molecules called surfactants, work much like dish detergent, helping clean up oil spills by breaking oil blobs into tiny droplets. Natural microbes in the ocean can then gobble up the droplets more easily. But for a massive spill like the Deepwater Horizon, scientists are wondering whether the chemical could undermine that process over time.
Right now, the biggest dispute between the government and BP over dispersants concerns their toxicity. Using planes and subs, BP crews have applied more than 37,850 liters of the chemical to the gulf each day over the past month, a small portion of it in the deep ocean. But neither the government nor BP expected the gusher to spew oil for this long, and concerns over the toxic effects of the dispersants have been growing. In the past few days, the government has pressured BP to scale back its dispersant use on the ocean surface and to look for less-toxic alternatives; the government said it will also look for more benign alternatives.
In lab tests, toxicologists have found that concentrations of Corexit, BP's dispersant of choice, can kill shrimp or fish. But the real-world effects remain unclear, as no one has ever spread this much dispersant in one place before. The tussle, and the studies, continue.
But there's another risk apart from toxicity, says David Valentine, a biogeochemist at the University of California, Santa Barbara: A chemical may undermine the cleanup effort's own microbial allies.
A few dozen microbes eat oil in the ocean, but they do it in different ways. The most potent oil-eaters are organisms such as Alcanivorax borkumensis, which are relatively rare in the ocean but have evolved to eat hydrocarbons from naturally occurring oil seeps. After spills, says Valentine, Alcanivorax tends to be the dominant microbe found near the oil. It secretes its own surfactant molecules to break the oil into round circles called micelles, then bonds to them and eats up the hydrocarbons.
Other microbes don't make surfactants but devour oil that's in small enough globs for them to eat. Valentine says a number of simple lab studies have shown that feeding those microbes with dispersed oil works by effectively breaking their food up. "I'm sure that the dispersant will allow, at first, more microbial degradation of the oil," he says.
What scientists don't know is how, over time, the surfactants in dispersants like Corexit might affect the ability of Alcanivorax and other surfactant-makers like it to eat oil. Could the dispersants bind to the micelles in a way that would hinder the microbes from doing so? Could they affect the way the microbe makes its own surfactant molecules? Might the surfactants in the Corexit interact somehow with the natural surfactants and render them less effective? "We really know very little about how these microbes work," says Valentine.
The National Science Foundation has given a $110,000 grant to Valentine to study the problem. He'll be joining a scientific cruise on 10 June in the gulf aboard the R/V Cape Hatteras. There, he plans to study the microbes feasting on the oil spill and use advanced methods of chromatography to analyze thousands of constituents of oil found in the water, on the surface, and in the deep.
*This article has been corrected. The original item stated that BP crews have applied more than 37,850 liters of the chemical to the gulf over the past month. They have actually applied that amount each day for that past month.