Natural microbial degradation plays an important role in the remediation of crude oil released to the marine environment. One important characteristic of the Deepwater Horizon oil release that could limit microbial degradation rates is that the oil is surfacing in waters that are highly depleted in available nutrients; microbes need these nutrients in order to grow and degrade oil. Although there have been comparatively few studies on N and P limitation of oil degradation since the 1980s, in the intervening time there have been major advances in our ability to assess the identity, activity and nutrient-limitation status of microbes in seawater; most of the early work centered on simply tracking the disappearance of oil over time, but now we have the ability to monitor the behavior of the microbial community in parallel.
With funding through this Grant for Rapid Response Research (RAPID), a team of microbial biogeochemists at the Woods Hole Oceanographic Institution will examine how microbial heterotrophy, nutrients, and petroleum compounds in the oil spill interact. They posit that the obvious potential for nutrient limitation to impact the degradation of oil from the Deepwater Horizon release warrants an immediate, targeted assessment of this issue in the Gulf of Mexico using up-to-date microbial and molecular methods. The study will achieve three objectives: 1) assess the degree to which microbes in the Gulf of Mexico are stressed by the unavailability of nutrients both inside and outside of areas contaminated by Deepwater Horizon oil; 2) examine the relationship between nutrient stress and the microbial degradation rate of Deepwater Horizon oil; 3) specifically determine whether oil degradation rates are stimulated by lecithin, a plant-derived, non-toxic, hydrophobic N- and P-containing organic nutrient.
Broader impacts The Deepwater Horizon oil release is an event of historic significance that has the potential to severely negatively impact the environmental quality and economic competitiveness of our nation. The primary benefit of the proposed work will be to reveal potential biogeochemical barriers to microbial oil degradation that might be overcome through novel remediation efforts. In addition, by conducting measurements collected by the lead investigator in 2002, the project will have the secondary benefit of identifying perturbations in the phosphorous field that will be propagated throughout the food web, including Gulf of Mexico fisheries.
More than a year after the largest oil spill in history, perhaps the dominant lingering question about the Deepwater Horizon spill is, "What happened to the oil?" A team from, Woods Hole Oceanographic Institution (WHOI) has come up with answers that represent both surprisingly good news and a head-scratching mystery. In research published on August 3, 2011 in Environmental Research Letters, the WHOI team studied samples from the surface oil slick and surrounding Gulf waters. They found that bacterial microbes inside the slick degraded the oil at a rate five times faster than microbes outside the slick—accounting in large part for the disappearance of the slick some three weeks after Deepwater Horizon’s Macondo well was shut off. At the same time, the researchers observed no increase in the number of microbes inside the slick—something that would be expected as a byproduct of increased consumption, or respiration, of the oil. In this process, respiration combines food (oil in this case) and oxygen to create carbon dioxide and energy. "What did they do with the energy they gained from this increased respiration?" asked WHOI chemist Benjamin Van Mooy, senior author of the study. "They didn’t use it to multiply. It’s a real mystery," he said. Van Mooy and his team were nearly equally taken aback by the ability of the microbes to chow down on the oil in the first place. Going into the study, he said, "We thought microbe respiration was going to be minimal." This was because nutrients such as nitrogen and phosphorus—usually essential to enable microbes to grow and make new cells—were scarce in the water and oil in the slick. "We thought the microbes would not be able to respond," Van Mooy said. But the WHOI researchers found, to the contrary, that the bacteria not only responded, but did so at a very high rate. They discovered this by using a special sensor called an oxygen optode to track the changing oxygen levels in water samples taken from the slick. If the microbes were respiring slowly, then oxygen levels would decrease slowly; if they respired quickly, the oxygen would decrease quickly. Bethanie Edwards, a biochemist in Van Mooy’s lab and lead author of the paper, said she too was "very surprised" by the amount of oil consumption by the microbes. "It’s not what we expected to see." She added that she was also "a little afraid" that oil companies and others might use the results to try to convince the public that spills can do relatively little harm. "They could say, ‘Look, we can put oil into the environment and the microbes will eat it,’" she said. Edwards, a graduate student in the joint MIT/WHOI program, pointed out that this is not completely the case, because oil is composed of a complex mixture molecules, some of which the microbes are unable to break down. "Oil is still detrimental to the environment, " she said, "because the molecules that are not accessible to microbes persist and could have toxic effects." These are the kinds of molecules that can get into the food web of both offshore and shoreline environments, Edwards and Van Mooy said. In addition, Edwards added, the oil that is consumed by microbes "is being converted to carbon dioxide that still gets into the atmosphere." Follow-up studies already "are in place," Van Mooy says, to address the "mysterious" finding that the oil-gorging microbes do not appear to manufacture new cells. If the microbes were eating the oil at such a high rate, what did they do with the energy? Van Mooy, Edwards, and their colleagues hypothesize that they may convert the energy to some other molecule, like sugars or fats. They plan to use "state-of-the-art methods" under development in their laboratory to look for bacterial fat molecules, a focus of Van Mooy’s previous work. The results, he says, "could show where the energy went." Van Mooy said he isn’t sure exactly what fraction of the oil loss in the spill is due to microbial consumption; other processes, including evaporation, dilution, and dispersion, might have contributed to the loss of the oil slick. But the five-fold increase in the microbe respiration rate suggests it contributed significantly to the oil breakdown. "Extrapolating our observations to the entire area of the oil slick supports the assertion microbes had the potential to degrade a large fraction of the oil as it arrived at the surface from the well," the researchers say in their paper. Edwards added that the results suggest "that microbes had the metabolic potential to break down a large portion of hydrocarbons and keep up with the flow rate from the wellhead." Also participating in the study from WHOI were researchers Christopher M. Reddy, Richard Camilli, Catherine A. Carmichael, and Krista Longnecker.
