Gulf's future depends on oil-eating bacteria, lingering toxicity

of Mexico,” 16 July 2008 HSNW). Valentine’s chief concern is that hypoxia could slow or alter oil metabolism in the dead zone such that oil lingers indefinitely. “We need to monitor oxygen levels very closely,” he says. “And a nagging concern is that they could suddenly plummet. Adult fish that run into [areas of] depleted oxygen can swim the other way, but many other species don’t have that option.”

Toxic constituents

Schmidt writes that Teske does not think that oil-spill-related hypoxia will be so dire. “Many people compare oxygen depletion in the oil plumes to the dead zone at the mouth of the Mississippi River,” he says. “But in reality, the two phenomena are quite different. The dead-zone hypoxia is much more severe; it consumes all the oxygen in the water and kills all marine life, while in the Deepwater Horizon plumes, you see much lower declines.” Compared with the almost 35 percent oxygen drop that Valentine has measured in the Gulf’s oil plumes, oxygen levels drop by about 90 percent in the Mississippi River dead zone.

So rather than oxygen levels, the long-term impact of oil on ecosystems long after it disappears from the water is what Teske finds more worrisome. This is because oil’s toxic constituents, such as polycyclic aromatic hydrocarbons, can disrupt reproduction of marine organisms and can lower their offsprings’ vitality.

This chronic toxicity will be magnified along the Gulf Coast’s beaches, salt marshes, and wetlands, because oil degradation in these sites will proceed at a much slower pace than in oxygen-rich environments, experts say. Oxygen levels plummet to near zero in sand deeper than 10 or 15 cm, whereas muddier sediments become hypoxic below just 2 or 3 cm, Florida State’s Huettel says. And oil that hits coastal areas has typically weathered into tar balls, which have low surface-area-to-volume ratios. When waves bury these tar balls under sediments, the conditions present a double whammy: Slow anaerobic bacteria must confront a dense, coagulated product with very stable aromatic carbon bonds that resist digestion. The microbes “just throw up their hands with this stuff,” UNC’s Teske says.

Long-term effects

Huettel and Kostka, who collaborate on coastal studies of microbial oil degradation, have recently found tar balls in sediments 50 cm deep in Florida beach sands. “It would take a hurricane to remove them,” Huettel says. Scientists worry that buried oil will release toxic compounds for decades.

Schmidt quotes UC Santa Barbara’s Valentine to say that no matter what cleanup techniques crews deploy, the Gulf’s future is a mixed bag. “Microbes will consume the oil in their own way, on their own time frame, with their own requirements,” he says. “And there will always be residual oil that the microbes can’t deal with. It’s going to be a long time before this system fully recovers.”