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July 28, 2010
Updated August 2, 2010, 3:10 P.M.
DOI: 10.1021/CEN072210151156

Microbes To The Rescue?

Oil Spill: The fate of spilled oil in the Gulf rests with the hydrocarbon-digesting microbes colonizing underwater plumes

Charles Schmidt

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MORE BELOW As the world's largest oil skimmer cleans up the Gulf’s surface, microbes break down millions of gallons of oil stuck underwater. UPI/A.J. Sico. View Enlarged Image
MORE BELOW As the world's largest oil skimmer cleans up the Gulf's surface, microbes break down millions of gallons of oil stuck underwater.

Millions of gallons of oil now drift throughout the Gulf of Mexico in massive, underwater plumes. Last week, scientists from the University of South Florida confirmed that this submerged oil came from BP's leaking well more than 5,000 feet below the ocean's surface. Over the past three months, the high pressure at the wellhead has pulverized the oil, while chemical dispersants have broken it into microscopic droplets. Unlike surface oil slicks, which physical forces such as evaporation can degrade, the fate of these hovering oil clouds—and by extension the Gulf's ecological future—lies chiefly with a biological process: the conversion of hydrocarbons into less harmful byproducts by marine bacteria.

After an April 20 explosion sank the Deepwater Horizon exploration rig and started the largest oil spill in U.S. history, microbiologists have converged on the Gulf to investigate the microbial composition throughout its waters and affected coastlines. According to Joel Kostka, a microbial ecologist at Florida State University, Tallahassee, the fundamental goals of this research are to determine the oil's impact on the Gulf's microbial ecosystems and to assess how limiting factors, chiefly oxygen concentrations, influence microbial oil degradation. Results could supply insights not only into the Gulf's ongoing recovery, Kostka says, but also into how scientists might direct cleanup operations more efficiently.

Preliminary data collected by these researchers show that marine microbes have mobilized across the Gulf and are in fact chewing their way through the oil plumes. These monitoring efforts have chiefly focused on drops in dissolved oxygen levels, a sign of microbial metabolism. Graduate students working with Andreas Teske, a marine biologist at the University of North Carolina, Chapel Hill, collected water samples from May 26 to June 8 aboard the University of Miami's research vessel (RV) Walton Smith. They found that alkane-digesting bacteria have colonized the Deepwater Horizon oil plumes and have begun to consume significant amounts of dissolved oxygen. Likewise, David Valentine, a microbial geochemist at the University of California, Santa Barbara, has observed microbe-associated oxygen declines in plumes of oil and methane gas. In these "gassy" plumes located within a 5- to 7-mile radius of the wellhead and at depths greater than 2,500 feet, oxygen levels drop by between 5 and 35%, he says. Valentine gathered his samples from June 11 to June 20, while aboard the RV Cape Hatteras, operated by the Duke/University of North Carolina Oceanographic Consortium. Also in a July 23 report, the government's Joint Analysis Group described dissolved oxygen drops at depths below 1,000 meters near the wellhead, where BP crews have injected dispersants directly into the leaking oil.

But scientists don't exactly know yet which bacteria species are present in these plumes. The Gulf has a "leaky" seafloor, populated with natural seeps that discharge between 560,000 to 1.4 million bbl of crude oil ever year, according to a 2003 National Research Council report on oil spills. Also hydrocarbons in general are ubiquitous in the ocean and can be found not only in seeping oil, but also in plant waxes and lobster shells. Myriad marine bacteria have evolved to consume these hydrocarbons, and now the spill has allowed them to travel beyond their natural food sources.

Teske is culturing bacteria obtained from Deepwater Horizon oil plumes to find out which bacteria are consuming the oil and how fast. He says millions of bacterial species can degrade hydrocarbons, a few of which rely on hydrocarbons alone as a carbon source. Also these oil plumes' microbial make-up will change over time as different genera degrade the oil's various fractions, says Markus Huettel, an oceanographer at Florida State University, Tallahassee. One specialist genus, Alcanivorax, appears on the scene first: It goes from being nearly undetectable in the absence of oil to overtaking all other microbes in oil-treated environments. Alcanivorax metabolizes branched-chain alkanes, which are oil's lightest fractions. As the alkanes disappear, Alcanivorax's numbers drop and then other bacteria take over to finish off what's left. One late-arriving genus, Cycloclasticus, consumes aromatic hydrocarbons, the next heaviest oil fraction after alkanes. But no microbe genus can digest oil's heaviest fraction, the asphaltenes.

STUCK IN SAND Oil tar balls buried in the Gulf's sandy coastline could persist for decades, leeching toxic chemicals. Joel Kostka
STUCK IN SAND Oil tar balls buried in the Gulf's sandy coastline could persist for decades, leeching toxic chemicals.

Whatever genera are present, says Joseph Suflita, a microbiologist at Oklahoma University, Norman, "the most important thing is that oil-degrading bacteria have the oxygen and nutrients they need to do the job." Oxygen is crucial because without it, anaerobic species, which use sulfate in place of oxygen during respiration, replace aerobic bacteria. These sulfate-dependent bacteria provide a slower path to oil degradation than aerobic microbes, Florida State's Kostka says. In laboratory tests performed by his graduate students, aerobic bacteria cultured from spill-impacted beaches along the Gulf coast cleared oil from water solutions in a day, while anaerobes took weeks to achieve the same effect. But UC Santa Barbara's Valentine says that the oxygen reductions he has observed aren't great enough to block the activity of aerobic microbes.

With these favorable conditions, aerobic bacteria could clear Gulf waters of the oil spill in a couple of years, UNC's Teske predicts. The use of chemical dispersants—which break the oil up into micrometer-sized droplets—is accelerating that process, he says, because surface-area–to-volume ratios increase with decreasing particle size

Although microbes may degrade the oil quickly, Valentine points out that they could eventually pose risks to the Gulf's ecosystem, particularly in the deep ocean. The introduction of a new food source, oil hydrocarbons, to the deep gulf could generate bacterial blooms that eventually become vast quantities of biomass. Other bacteria would then metabolize this biomass and deplete oxygen to levels low enough to be dangerous for other organisms, Valentine says. The Gulf of Mexico already suffers from an enormous hypoxic dead zone, measuring more than 8,000 sq miles, generated by nutrient-laden runoff from the Mississippi River. 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 depleted oxygen can swim the other way, but many other species don't have that option."

Teske doesn't 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 to the almost 35% oxygen drop that Valentine has measured in the Gulf's oil plumes, levels drop by about 90% in the Mississippi River dead zone.

So instead of oxygen levels, Teske worries more about the oil's long-term impact on ecosystems long after it disappears from the water: Oil's toxic elements such as polyaromatic hydrocarbons can disrupt marine organism reproduction and can lower their offspring's' vitality. "If you remove one or two generations from a species' reproductive cycle," he says, "it will take a long time to recover."

And this chronic toxicity will be magnified along the Gulf coast's beaches, salt marshes, and wetlands, because oil degredation will proceed at a much slower pace, experts say. Oxygen levels plummet to near zero in sand deeper than 10 or 15 cm, while muddier sediments become hypoxic below just 2 or 3 cm, Florida State's Huettel says. And oil that washes ashore is typically weathered into tar-balls, which have very low surface-area to volume ratios. These conditions are 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.

Huettel and Kostka, who collaborate on coastal studies of microbial oil degredation, have recently found tar balls in sediments 50 centimeters deep in Florida beach sands. "It would take a hurricane to remove them," Huettel says. Scientists worry most that buried oil will secrete toxins for decades. Researchers have observed a similar time course in Prince Williams Sound, Alaska—the scene of the 1989 Exxon Valdez spill—and other coastal areas hit by major spills.

To accelerate oil degradation, some scientists have proposed spraying wetlands with microbial nutrients, such as nitrogen, phosphorous, and iron. But Huettel cautions that approach could be problematic. The nutrients also stimulate algal blooms, which can lead to dead zones, he says. Also Kostka says that use of these microbial "fertilizers" in Prince Williams Sound produced diminishing returns. At first, the fertilizers sped up the recovery process, he says, but within five years, differences between treated and un-treated beaches were negligible, and eventually the treated beaches recovered at a slower rate. "The fertilizers upset the system's natural ecological balance," Kostka says. "You have to be very careful; their harms can sometimes outweigh their benefits." The better approach, Kostka says, is to let science direct cleanup and be more proactive. As oil continues to wash ashore from the Gulf's waters, local environmental managers could strategically deploy booms where oil is more likely to be trapped by hypoxic sediments, he says.

No matter what clean-up techniques that crews deploy, UC Santa Barbara's Valentine predicts 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."

Chemical & Engineering News
ISSN 0009-2347
Copyright © 2011 American Chemical Society
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