Dispersant May Have Made Effects of Deepwater Horizon Spill Much Worse

What was once considered a reliable, strategic effort to mitigate the damage caused by a catastrophic oil spill has now become a case study in the risks of messing with microbes.

In 2010, the Deepwater Horizon oil rig exploded in the Gulf of Mexico, releasing approximately 4.2 million barrels of oil into the water. It’s often cited as the largest accidental marine oil spill in the history of the petroleum industry.

As part of the clean up process, authorities dumped 1.8 million gallons of a dispersant called Corexit into the Gulf, which was intended to break apart the slick into scattered clumps. This way, oil doesn’t wash onto shores in a mat or harm coastal wildlife.

A plane disperses Corexit dispersant onto the Gulf of Mexico.

But to be successful, degradation, as it’s called, requires that oil-digesting microbes break those clumps down further. These microbes occur naturally—they’re accustomed to digesting hydrocarbons that bubble out of deep-sea vents. In essence, the spill would give them another food source, which the dispersants would make more accessible by increasing the surface area on which they can feed.

At least, that’s what’s supposed to happen. But a new study by Samantha Joye at the University of Georgia shows that instead of accelerating the microbes’ ability to degrade oil, Corexit actually impeded it. She and her colleagues took a sample of water from a naturally hydrocarbon-rich Gulf site and simulated local conditions—including the effect of the Deepwater spill—back at the lab.

Here’s Ed Yong, writing for The Atlantic:

The team analyzed these microcosms with a battery of techniques. They counted microbial cells, and sequenced them to identify the species that were present. They quantified the enzymes made by those microbes to gauge how active they were. They checked the production of microbial molecules used to break down the oil. And they used slightly radioactive oil compounds to directly measure the rate at which those substances were being degraded. “No one has ever done all of that in a single experiment,” says Joye.

It appears that Corexit spurred the growth of Colwellia, a bacteria that originally represented 1% of the total microbes in their sample before the dispersant was added—but after, it represented 43%. In addition, Corexit suppressed the natural growth of Marinobacter, which kicked in only when Corexit was absent. These effects combined, the microbial mixture was rendered ineffective in degrading Deepwater oil.

If we had left bacteria in the Gulf alone, some scientists say, perhaps the situation would be better today.

While Joye’s results have been replicated, others remain skeptical. And some have argued for the introduction of nitrogen, which stimulates bacteria like Marinobacter. Still, it may often be wiser to “let nature take its course” rather than mess with misunderstood ecosystems.