Energy and environment - biological decomposition

The job of cleaning up after the oil spill in the Gulf of Mexico will fall to the bacteria

By David Baylo

Pumps, sweepers and tens of kilometers of floats can't compete with bacteria and other microscopic organisms when it comes to removing oil. The microscopic organisms that naturally inhabit the Gulf of Mexico are the only real defense against the oil spill from the Deepwater Horizon drilling rig. As scientists study how the bacteria clean up the filth, they also fear that these saviors could just as well be suffocating marine life in the bay.

The fact that the bacteria in nature are more effective than human clean-up efforts may come as a surprise after decades in which researchers have tried to create oil-devouring super-bacteria using genetic engineering methods. In fact, the first patent registered for a genetically modified organism was for a bacterium that feeds on hydrocarbons. But these bacteria "are largely ineffective," says Jay Grimes, a marine microbiologist at the University of Southern Mississippi.

Engineered bacteria are not up to the task partly because there is no single creature, no matter how upgraded, that can compete with the power of an entire community of different creatures, each of which has a specific specialization in hydrocarbon consumption. "In every ocean we study, from the Antarctic to the Arctic Ocean, we find oil-breaking bacteria," says Ronald M. Atlas, a microbiologist at the University of Louisville. Atlas tested and evaluated, among other things, the ability of engineered bacteria to clean up the oil spill from the Exxon Valdez tanker. "Crude oil is a complex mixture of thousands of compounds, and the communities that feed on it are also complex. Superbugs fail because they compete with these environmentally well-adapted communities.”

And if you can't beat them, why not take advantage of them? Theoretically, these creatures can be stimulated to work harder with fertilizers, such as iron, nitrogen and phosphorus. In fact, such an approach accelerated bacterial activity in the soil along the Alaskan coast after the Valdez disaster. "We saw a three- to five-fold increase in the rate of biodegradation," Atlas recalls.

But it seems that it will not pay to use this method in the Gulf of Mexico. "How can we make sure that the fertilizer stays with the oil in the ocean?" asks ecological microbiologist Kenneth Lee, director of the Center for Offshore Oil, Gas and Energy Research at the Government of Canada's Department of Fisheries and Oceans. And he says: "We don't see biological restoration activity in the middle of the sea." And even fertilization along the coast is problematic. Lee tried to cultivate oil-saturated wetlands in Nova Scotia, but their oxygen supply is too low to allow an increase in bacterial activity. "It didn't go well. Instead we achieved weathering and erosion of the soil on a wide scale."

Chemicals that break the oil slick into small droplets, like an emulsion, are perhaps the only way to speed up bacterial activity in the bay. "When the oil is dispersed in very small droplets, the bacterial degradation is much faster," says Lee, who took measurements of the oil droplet size to determine the effectiveness of the dispersant chemicals.

But the oil consumption of the bacteria is high only close to the surface area of ​​the warm water. "The metabolism slows down by about two or three times for every 10 degrees Celsius drop in temperature," says biochemist David Valentine of the University of California, Santa Barbara, who studies the response of bacteria in the Gulf to the oil spill. "Decomposition of oil in the depths of the sea," he says, "will be very slow due to such a low temperature."

And unfortunately, this is exactly where some of the crude oil that leaked from the rig ends up. Researchers have seen oil "between 800 and 1,400 meters deep," says ecological microbiologist Andreas P. Teske of the University of North Carolina at Chapel Hill. His research student, Luke McKay, was aboard the Pelican research vessel that first reported these underwater oil clouds. In the depths, bacterial activity is the only process for breaking down oil. (On the surface, physical processes such as evaporation and the movement of waves help remove the oil from the water.)

But there is also a dark side to the activity of bacteria that devour large quantities of oil such as those that leaked in the Gulf of Mexico. When they break down the oil, they consume the oxygen dissolved in the water, an action that can suffocate aerobic organisms that need oxygen. Measurements of the oxygen concentration in the seawater of the Gulf of Mexico showed a drop of up to 30% in the weeks following the spill. Although such a decrease in level has almost no effect on mobile marine creatures, scientists fear the effect of the lack of oxygen at depth, in areas where the water hardly mixes with the surface water, which is rich in oxygen. This is bad news both for the continuation of the rapid decomposition of the oil and for the well-being of the corals and other creatures anchored on the bottom. Even now the corpses of dead sea cucumbers floated up from the bottom, possibly hinting at the development of a dead zone and also at the toxic effect of the oil itself.

To understand how the bacteria will do their work and for how long, it is necessary to know the amount of crude oil found in the sea. Such an estimate "depends on the size (of the spill), and we don't know how big it is," says microbiological geochemist Samantha B. Joy of the University of Georgia. "We can't make any kind of calculation about possible oxygen consumption or anything else." Over time, the leak estimate jumped from 1,000 barrels of oil per day, initially, to 60,000 per day in mid-June 2010.

Either way, this oil will be around for a long time. Microscopic organisms break down hydrocarbons "over weeks to months to years," Atlas explains. Nature provides a solution, but a slow one.