OXYGEN Depletion

Already, measurements of oxygen depletion of as much as 30 percent in the Gulf of Mexico seawater suggest that the microbes are hard at work eating oil. "I take the 30 percent depletion of oxygen in water near the oil as indicating bacterial degradation," Atlas says.

That happens best near the surface, whether at land or sea, where warm-water bacteria such as Thalassolituus oleivorans can thrive; colder, deeper waters inhibit microbial growth. "Metabolism slows by about a factor of two or three for every 10 degree[s] Celsius you drop in temperature," notes biogeochemist David Valentine of the University of California, Santa Barbara, who just received funding from the National Science Foundation to characterize the microbial response to the ongoing oil spill. "The deeper stuff, that's going to happen very slowly because the temperature is so low."

Unfortunately, that's exactly where some of the Deepwater Horizon oil seems to be ending up. "They saw the oil at 800 to 1,400 meters depth," says microbial ecologist Andreas Teske of the University of North Carolina at Chapel Hill, whose graduate student Luke McKay was on the research vessel Pelican that first reported such subsurface plumes—as predicted by small-scale experiments, such as the U.S. Minerals Management Services Project "Deep Spill". "It is either at the surface or hanging in the water column and possibly sinking down to the sediment."

Yet, microbes are the only process to break down the oil deeper in the water, far away from physical processes on the surface such as evaporation or waves. "The deep waters are dominantly microbial" when it comes to oil degradation, although these communities are not as well studied as those at the surface, notes microbial geochemist Samantha Joye of the University of Georgia. "As long as there is oxygen around, it will get chewed up."

To understand how the microbes will work and how quickly, however, will require a better understanding of exactly how much oil is out there. "It's a function of size, and we don't know size," Joye says. "We need to know how much oil is leaking out. Without that information we can't begin to make any kind of calculation of potential oxygen demand or anything else." BP now admits that its original estimate of roughly 200,000 gallons per day was far too low without providing an alternative; independent experts have offered estimates as high as four million gallons per day.

It is possible to add fertilizers, such as iron, nitrogen and phosphorus, to stimulate the growth of such bacteria, an approach used to speed up microbial activity in the sediment along the Alaska coast after the Exxon-Valdez spill. "We saw a three to five times increase in rate of biodegradation," Atlas says, suggesting the technique might prove effective along the oil-inundated Louisiana coast as well. "It was hundreds of miles of shoreline, the largest bioremediation project ever."

But that's strictly onshore. "In the ocean, how do you keep the nutrients with the oil?" Lee asks. "It's much easier to add to soil. That's why you don't see bioremediation in the open ocean." And aerating soils in wetlands can have its own problems; Lee tried tilling oil-soaked wetlands in Nova Scotia where there was limited oxygen to increase microbial activity. "That didn't work. We had large erosion as a result," he says. "If the oil reaches shore, our recommendation was to leave the oil alone and let nature do it."

But sediment, whether the muck of Louisiana marshland or the deep ocean seafloor, suffers from a dearth of oxygen. That means it's up to anaerobic microbes—ancient organisms that live via sulfate rather than oxygen—to do the dirty work of consuming the spill. "What occurred in 10 days aerobically, took 365 days to occur anaerobically," says Atlas of the breakdown of oil in the wake of the Amoco Cadiz spill off the coast of France in 1978. Adds Teske: "The heavy components are sinking to the sediment and forming an oily or tarry carpet or getting buried. Then they are much harder to degrade."

Such anaerobic environments can develop locally in the seawater itself, thanks to a ready supply of oil and blooming microbes eager to devour it. In deepwater, where there's less mixing with the surface waters to provide fresh supplies of oxygen, a dead zone may result. "It's not exchanging with the atmosphere," Joye notes. "Once the oxygen is gone, how are you going to replace it? It's not going to get mixed up by winter storms." That's bad news for the speedy breakdown of oil as well as for the Lophelia coral and other sessile deepwater life.

At the same time, the addition of 130,000 gallons of dispersants deep beneath the surface is having uncertain effects; it may even end up killing the microbes it is meant to help thanks to the fact that Corexit 9527A contains the solvent 2-butoxyethanol, which is a known human carcinogen and toxic to animals and other life. But the U.S. Environmental Protection Agency, National Oceanic and Atmospheric Administration and others are monitoring whether adding such dispersants ends up boosting microbe-growth and hence dangerously depletes oxygen levels, among other potential environmental ill effects.

Nor is it clear how fast the microbial community will respond. "Which microbial communities are the fastest responders?" Teske asks. "That would be interesting to know" and this oil spill may provide the real- world answer. Some research suggests that oil spills may actually feed themselves nitrogen by stimulating the growth of various bacteria that fix the vital nutrient, Joye notes. At the same time, microbial predators such as protozoa tend to dampen the efficiency of would-be oil-eating microbes.