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Gulf oil spill was abated by bacteria that feed on hydrocarbons, report says

By Brian Palmer
Tuesday, January 11, 2011; 12:31 AM

The Deepwater Horizon oil spill in the Gulf of Mexico was, by far, the biggest environmental story of 2010. We've turned our minds and diaries to 2011, but nature doesn't abide by the Gregorian calendar. What's going on with the nearly 5 million barrels, or 207 million gallons, of oil that leaked? Will the gulf ever be oil-free again?

No, but the vast majority of the oil from the Deepwater Horizon spill is already gone, according to a final government report released last month. That report, along with several experts contacted by the Lantern, provides a detailed picture of the oil spill and its aftermath.

Trying to remove all oil from the gulf would be a Sisyphean task. Every year, oil tankers, drilling platforms and boats spill more than 310,000 gallons of oil into the gulf. But even if we halted human activity in the gulf, natural seeps from the sea floor would still send 42 million gallons of oil into gulf waters each year.

These seeps actually prevented the BP spill from being an even worse disaster. The gulf has more natural seeps than any other body of water in or around North America. Because of this constant supply of hydrocarbons, there is always a healthy population of bacteria floating around the gulf looking for more food. When BP's well blew out, these tiny creatures went into a feeding frenzy. (The lack of natural seeps, and oil-eating bacteria, is one of the reasons that Alaska's Prince William Sound has been slow to bounce back from the Exxon Valdez disaster of 1989.)

Bacteria are far more reproductively agile (so to speak) than mammals, which take years to adjust their population size to changes in food supply. Within 24 hours of the spill, the number of oil-eating bacteria around the wellhead had grown tenfold.

Under the right conditions, bacteria can degrade spilled oil remarkably quickly. Fortunately, conditions in the Gulf of Mexico are quite friendly to oil-gobbling bacteria, and the overwhelming majority of the BP oil was recaptured, burned off, evaporated, flushed out by ocean currents or eaten by bacteria by the end of July, just two weeks after BP managed to seal off the well. Because the bacteria and cleanup crews worked so quickly and the spill is now so diluted, it's impossible to say exactly what percentage of the original oil is still around, but it's a small fraction.

When will every drop of the spilled oil be gone? It's hard to say. A gallon of oil contains a wide variety of compounds. Even hydrocarbons, the basic building blocks of oil, come in thousands of varieties. Different types of bacteria feed on different kinds of hydrocarbons, and some work faster than others. In addition, a single bacterial cell doesn't take in a drop of hydrocarbon, fully digest it and excrete carbon dioxide and water. It's more of a team effort, with several intermediate compounds being passed between hungry microbes.

Bioremediation experts talk about half-life, by which they mean the time it takes for the bacteria to break down half of the oil. If you can keep cutting the amounts in half, you'll never get to zero, but you'll quickly get to inconsequential concentrations. Those last few molecules of hydrocarbons - the chemical compounds in oil that cause most of its health risks - aren't particularly important when they're diluted in the gulf's 643 quadrillion gallons of water.

Researchers have used a bunch of fascinating strategies to measure biodegradation rates, both in the open water and in the laboratory. For example, they have set so-called bug traps - saturated beads of oil that attract bacteria - in the gulf. They watch which bacteria make their way to the oil and how long it takes them to break it down. By most estimations, the right colony of bacteria can break down oil so quickly that within six days the hydrocarbons are undetectable using the best available instruments.

Between July and August, researchers from the Lawrence Berkeley National Laboratory took 170 samples of gulf water, starting at the wellhead and following the oil's likely path away from it. They couldn't find any spilled BP oil, which means the concentration was less than two parts per billion. (These researchers and others have developed techniques to "fingerprint" oil, so they can tell fairly accurately whether oil found in the gulf came from the BP well or elsewhere.)

Another team of researchers, from the Woods Hole Oceanographic Institution, reported that underwater plumes of oil - that is, oil that hadn't reached the surface - may be breaking down at a slower rate. But these researchers haven't found many plumes in the last six months, suggesting that they probably aren't having a significant ecological impact at this point.

It's too soon to declare a happy, green ending to this story. We don't really know how the chemicals that bacteria release after eating oil will affect human and animal health. Some of them are known to mimic testosterone and estrogen. It's possible, although somewhat unlikely, that these compounds hamper endocrine functions at exceedingly low concentrations.

Another concern is oil that has been trapped in sediment. Bacteria in open water work much more efficiently than their mud-based cousins, because the latter have less access to oxygen and nutrients. That's one of the many reasons the spill response was so focused on preventing the muck from making landfall. The bacteria in marshlands and other coastal areas are still breaking down oil. Recovery workers are experimenting with techniques to help them along, for example by using aeration tubes to pump in oxygen and nutrients.

And although the oil mostly disappeared several months ago, an untold number of fish, aquatic mammals and birds died in the disaster. It will probably take generations before their populations return to pre-spill levels. If only mammals could reproduce like bacteria.

The Lantern thanks Terry Hazen of the Lawrence Berkeley National Laboratory and Joe Lepo of the University of West Florida for their assistance. Is there an environmental quandary that's been keep you up at night? Send it to ask.the.lantern@gmail.com.

The Green Lantern is a weekly environmental column from Slate.

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