Like cars, some microbes use oil as fuel. Such microorganisms are a big reason why BP's 2010 oil spill in the Gulf of Mexico was not far worse.
"The microbes did a spectacular job of eating a lot of the natural gas," says biogeochemist Chris Reddy of the Woods Hole Oceanographic Institution. The relatively small hydrocarbon molecules in natural gas are the easiest for microorganisms to eat. "The rate and capacity is a mind-boggling testament to microbes," he adds.
As Reddy suggests, the microbes got help from the nature of the oil spilled—so-called Louisiana light, sweet crude mixed with natural gas, as opposed to bitumen or other heavy, gunky oils. "It's a whole lot easier to degrade," says Christopher D'Elia, a biologist at Louisiana State University and dean of the School of the Coast and Environment. "The bacteria had something that was more tractable."
More than 150 different molecules make up the toxic stew of hydrocarbons that spewed from BP's Macondo well on the Gulf of Mexico seafloor. The microbes chewed through the smaller, dispersed hydrocarbons (and the dispersants themselves) relatively quickly, helped by the fact that these molecules can dissolve in water. "I give them a 7 out of 10," says biogeochemist David Valentine of the University of California, Santa Barbara, of the microbes’ performance eating the oil spill.
Ocean currents, in addition to keeping the spilled oil offshore, spurred microbial activity amidst the oil spill. That continuous mixing of the water allowed a bacterial bloom to turn millions of barrels of oil into an estimated 100 sextillion microbial cells of ethane-consuming Colwellia, aromatic-eating Cycloclasticus, alkane-eating Oceanospirillales, oil-eating Alcanovorax, methane-loving Methylococcaceae and other species, including at least one previously unknown to science.

But even the ravenous microbes could not clean it all—and much of what they consumed (natural gas components like methane, ethane, butane, propane and pentane) does not legally count as part of the oil spill. Plus, plenty of tarlike hydrocarbons—which are far too big for microbes to chew up—spilled, too. Reddy and his colleagues still head down to the Gulf of Mexico as often as possible to walk the beaches and collect samples. "We're trying to see who's the toughest kid on the block," he says of the spill’s components, in an attempt to figure out why these hydrocarbons cannot be biodegraded or even broken down by sunlight. In fact, sunlight alone can transform the oil that made it to the surface uneaten. "Nature has a vast toolbox to combat oil," he adds, although it remains unclear whether sunlight-transformed hydrocarbons are worse or better from a toxicology perspective.
The bacterial blooms also seem to be at least partially responsible for the oily marine snow that coated the bottom of the Gulf of Mexico in the wake of the Macondo blowout—again, an unexpected after-effect with unknown impacts. "The stuff is almost everywhere you look," says biogeochemist Samantha Joye of The University of Georgia, describing her surveys of the sediments under 1,000 meters or more of water with submersible Alvin, among other tools. "Up to 15 percent of what was discharged is on the seabed. That's a pretty remarkable number, given that it wasn't initially thought of as a potential fate for oil."
Even the smaller molecules cannot be consumed if there are not enough nutrients in the water as well, like nitrogen or phosphorus. "Nutrients regulated biodegradation," Joye says. "That could be why so much oil sedimented out, they degraded as much as they could." In fact, the microbes may have been hampered not only by limited nutrients because the microbial population boom may have meant an accompanying boom in their predators or in the various viruses that can infect these spill-eaters. Moreover, one of the biggest requirements for these microbes to eat hydrocarbons—oxygen—is not present at all in the sediments of the deep or the muck of Louisiana marshes. That is why oil from the Macondo well persists in those places five years later—and perhaps for eons to come.
"Microbes are like teenagers," Reddy says. "You can ask them to clean the garage over the weekend. Can they do it? Yes. Will they do it? Maybe. Will they do as good a job as you want? Probably not."