When the Deepwater Horizon oil well blowout spewed 5-million barrels of oil into the Gulf of Mexico, one of the main tools used against the oil was a chemical dispersant called Corexit. 7-million litres of this detergent-like chemical was used to break up oil slicks, in part to disperse the oil into the water and prevent contamination of coastlines, birds, and marine mammals.
It was also thought that dissolving the slicks like this would increase the rate at which natural bacteria would bio-degrade the oil. But work by Dr. Samantha Joye, a microbiologist in the Department of Marine Sciences at the University of Georgia, and her colleagues, has shown that Corexit seems to inhibit, rather than facilitate, the ability of microbes to break down oil, leaving the toxic oil in the water for longer.
This throws into question a big part of the case for using chemical dispersant on oil spills.
“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.
So if the oil wasn’t degraded by the bacteria, the question remains: Where did it go? Joye guesses it might still be on the floor of the gulf.
Should authorities avoid dispersants in the future? “That’s an extraordinarily complicated question,” says Joye. Corexit has its problems, but it does seem to keep oil away from coasts. “Nobody wants to see oiled birds, turtles, and dolphins, but the bottom line is that if you disperse that oil, it’s still in the water. You feel better, but is it improving the situation? My gut instinct is that I would put my faith in the microbial communities to do their job.”