For microscopic animals living in the Gulf, even worse than the toxic oil released during the 2010 Deepwater Horizon disaster may be the very oil dispersants used to clean it up, a new study finds.

More than 2 million gallons (7.5 million liters) of oil dispersants called Corexit 9527A and 9500A were dumped into the Gulf of Mexico in an effort to prevent oil from reaching shore and to help it degrade more quickly.

However, when oil and Corexit are combined, the mixture becomes up to 52 times more toxic than oil alone, according to a study published online this week in the journal Environmental Pollution.

"There is a synergistic interaction between crude oil and the dispersant that makes it more toxic," said Terry Snell, a study co-author and biologist at Georgia Tech. Using dispersants breaks up the oil into small droplets and makes it less visible, but, "on the other hand, makes it more toxic to the planktonic food chain," Snell told LiveScience.

Toxic mixture

That mixture of dispersant and oil in the Gulf would've wreaked havoc on rotifers, which form the base of the marine food web, and their eggs in seafloor sediments, Snell said.

In the study, Snell and colleagues tested ratios of oil and dispersant found in the Gulf in 2010, using actual oil from the well that leaked in the Deepwater Horizon oil spill and the dispersant. The mixture was similarly toxic at the various ratios tested, the study found. His group exposed several varieties of rotifers to concentrations of the oil-dispersant mixture likely seen over a large area of the Gulf.

"The levels in the Gulf were toxic, and seriously toxic," Snell said. "That probably put a big dent in the planktonic food web for some extended period of time, but nobody really made the measurements to figure out the impact." [Deepwater Horizon: Images of the Impact]

The dispersant makes the oil more deadly by decreasing the size of the droplets, making it more "bio-available" to small organisms, said Ian MacDonald, a researcher at Florida State University. "The effect is specifically a toxic synergy — the sum is worse than the parts," said MacDonald, who was not involved in the research.

A cautionary tale

This is one of the first studies to look at the impact of the oil-dispersant mixture on plankton. A decline in populations of plankton could impact larger animals all the way up to whales, he said. In general, plankton can rebound quickly, although the toxicity to larvae in sediments is concerning, since it reduces the size of the next generation. This ocean-bottom oil slurry could have also impacted other species that spend part of their life cycles here like algae and crustaceans.

"This is an important study that adds badly needed data to help us better understand the effects of oil spills and oil spill remediation strategies, such as the use of dispersants," said Stephen Klaine, an environmental toxicologist at Clemson University who wasn't involved in the research. "Species' differences in the sensitivity to any toxic compounds, including the ones in this discussion, can be huge."

The results contrast with those released by the Environmental Protection Agency in August 2010. That study found that a mixture of oil and Corexit isn't more toxic than oil alone to both a species of shrimp and species of fish. However, several studies have found the mixture is more toxic than oil to the embryos of several fish species. The EPA could not immediately be reached for comment.

"To date, EPA has done nothing but congratulate itself on how Corexit was used and avow they would do it the same way again," MacDonald said.

However, Snell said the dispersant should not be used. It would be better to let the oil disperse on its own to minimize ecological damage, he said.

"This is a cautionary tale that we need to do the science before the emergency happens so we can make decisions that are fully informed," Snell said. "In this case, the Corexit is simply there to make the oil disperse and go out of sight. But out of sight doesn't mean it's safe in regard to the food web."

"It's hard to sit by and not do something," Snell said."But in this case, doing something actually made it more toxic."

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