Exploring microbial and hydrocarbon dynamics along a surface transect of a persistent oil slick at Taylor Energy

Oil in surface slicks is subject to a variety of weathering processes, including photo-oxidation, evaporation, and biodegradation. The oil slick sourced from the site of the sunken Taylor Energy platform 23051, located 11 miles southeast of the mouth of the Mississippi River, offers investigators a unique opportunity to explore the co-evolution of oil and microbial communities at the sea surface in an oil-impacted setting. Chemical dispersants were originally designed to expedite biodegradation in surface oil spills, but their impact on the hydrocarbon degrading community and activity in the surface remains poorly constrained. Using microcosm incubations of surface water from 4 sites along a transect at the Taylor site surface slick, we explored how the addition of nutrients, of Corexit 9500, and the joint addition of nutrients and Corexit 9500 altered the microbial community and hydrocarbon degradation rates. We evaluated oil biodegradation efficiency by measuring turnover of radiolabeled hexadecane and naphthalene to carbon dioxide, and further assessment of the incubations through cell counts, nutrient, DOC, and extractable hydrocarbon profile analysis. We observed temporal and site specific variations in the turnover of naphthalene and hexadecane, with the in situ percent turnover/day of both hexadecane and naphthalene highest at the site farthest away from the leaking source. Our current results indicate that the surface slick is a dynamic system that exhibits varying responses to nutrients and dispersant additions.