Bioremediation of an Experimental Oil Spill in a Coastal Louisiana Salt Marsh

Since the 1970s, large-scale oil contamination of various marine ecosystems has been a source of much concern.  As our dependence on fossil fuels grows, so to has its environmental impact.  In 2010, an oil rig known as the Deepwater Horizon positioned off the coast of Louisiana in the Gulf of Mexico exploded, resulting in the largest oil spill in United States history.  Many scientists and environmentally savvy individuals thought the gulf was doomed; they assumed the resulting trickle-down of toxins through the ocean’s food web would devastate many populations and upset delicate coastal ecosystems.  To the world’s astonishment, though, most species have not suffered significant losses and only 18 months later, the amount of oil in the water has dropped to normal levels. Several studies that have taken place in the last year have shown that marine microorganisms are responsible for this speedy cleanup.  Academics have been aware of certain bacterial species that can digest oil since the first major spill nearly a half century ago, but lately the big question has been how can we stimulate the bacteria into digesting the hydrocarbons that make up crude oil faster than they do naturally?  This study investigated the possibility that microorganisms that live in close association with a marine plant, Spartina alterniflora, would break down oil faster in the presence of excess nitrogen-containing compounds (ammonium nitrate or urea).  By creating a simulated oil-spill and applying different treatments to certain areas of it, the researchers were able to test oil biodegradation rates against the control plots. Tate et al. conclude that saturating the soil with nitrogen does not accelerate biodegradation of crude oil and that oxygen is much more likely the environmental factor that affects the rate of uptake and digestion.—Edward McLean
         Tate T.,  Shin, W., Pardue, J., Jackson W. Bioremediation of an Experimental Oil Spill in a Coastal Louisiana Salt Marsh. Springer Science+Business Media B.V. 2011.

         For a growing number of years, proponents of in situ (at the source) bioremediation have recommended applying nutrients to an oil-contaminated area to accelerate the process of biodegradation.  However, only a limited amount of information exists on successful nutrient treatments that will have the same effect in different ecosystems, due to the uniqueness of each one’s biodiversity, its soil/water composition, and its overall chemistry.  No such information had yet been published with regards to proper nutrient amendment of salt marshes in the Gulf of Mexico, which Tate et al. set out to change.  Through experimental trial and error under laboratory conditions, they found that nitrogen was most likely to accelerate biodegradation; salt marsh soils contain many nitrogen fixing microbes that also naturally digest hydrocarbons, and both of these processes speed up in the presence of excess nitrogen.  For their study, they chose 10 blocks and then cut each block into quadrants.  Each quadrant received a distinct treatment and quadrants were randomized at every block: the control, which had no oil or fertilizer added, the second with oil, but no fertilizer, the third with oil and nitrate, and the fourth with oil and urea.  Tate et al. added 142 l of sweet Louisiana crude oil (SLCO) to their study site, but only after weathering most of the impurities out of the oil by moving air through the drum for 2 hours at a time.  The authors spent considerable time measuring the soil before their study, especially its nutrient and microbiological composition.  Once they had built up a baseline understanding of the properties of their study site, they began testing. 
         After meticulously setting up this experimental oil spill and accounting for a large range of potential variables, the results of this study must have brought about a certain amount of disappointment in Tate and his team.  In terms of changing the soil composition in the slight way they had desired, it was a success: ammonia concentration increased and the soil was generally more nitrogen-saturated.  But in terms of stimulating uptake and digestion of different hydrocarbons, there was no statistical difference in the plots that received fertilizer and those that did not, leading the authors to conclude that nitrogen is not the key environmental factor in accelerating biodegradation of crude oil.  Despite not finding a nutrient amendment that might benefit future environmentalists hoping to clean up a contaminated salt marsh faster, this study still managed to achieve a great deal: S. alternifloraand its symbiotic microbes have an incredible knack for removing hydrocarbons from the soil with their own biological machinery: it took between 100 – 200 days for most of the simulated contamination to be removed completely.  Continued research into this natural process could shed some much needed light on future bioremediation efforts that set out to clean up oil contamination.

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