Effects of COREXIT EC9500A on bacterial communities influenced by the Deepwater Horizon oil spill

Hydrocarbon-degrading bacteria are important to controlling the fate of natural and anthropogenic hydrocarbons in the marine environment and will be an important component to the natural attenuation of the Deepwater Horizon spill. The chemical dispersant COREXIT®EC9500A was widely deployed during the Deepwater Horizon response. Although toxicity tests confirm that COREXIT®EC9500A does not pose a significant threat to invertebrate and adult fish populations, there is limited information on its effect on microbial communities. Microbial community composition was determined in freshly deposited oil on a beach in Louisiana, resulting from the spill. Secondary heterotrophic production and viability in cultures obtained from oil samples was determined in the presence and absence of COREXIT®EC9500A . Vibrio isolates were abundant in length heterogeneity-PCR fingerprints of oil samples along with hydrocarbon-degrading isolates affiliated with Acinetobacter and Marinobacter. Significant reductions in Acinetobacter and Marinobacter production and viability in the presence of the dispersant compared to controls were observed. Marinobacter is most sensitive to the dispersant as evidenced by a near 100% reduction in viability and production as a result of exposure to environmentally relevant concentrations of the dispersant. Significantly, at the same dispersant concentration, non-hydrocarbon-degrading Vibrio isolates proliferate. These data suggest that hydrocarbon-degrading bacteria are inhibited by this dispersants and that it’s use could potentially diminish the capacity of environmental microbial communities to bioremediate the spill.