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PreprintChemical dispersants can suppress the activity of natural oil-degrading microorganisms( 2015-09) Kleindienst, Sara ; Seidel, Michael ; Ziervogel, Kai ; Grim, Sharon L. ; Loftis, Kathy ; Harrison, Sarah ; Malkin, Sairah Y. ; Perkins, Matthew J. ; Field, Jennifer ; Sogin, Mitchell L. ; Dittmar, Thorsten ; Passow, Uta ; Medeiros, Patricia M. ; Joye, Samantha B.During the Deepwater Horizon oil well blowout in the Gulf of Mexico, the application of 7 million liters of chemical dispersants aimed to stimulate microbial crude oil degradation by increasing the bioavailability of oil compounds. However, the effects of dispersants on oil biodegradation rates are debated. In laboratory experiments, we simulated environmental conditions comparable in the hydrocarbon-rich, 1100m deep, plume that formed during the Deepwater Horizon discharge. The presence of dispersant significantly altered the microbial community composition through selection for potential dispersant-degrading Colwellia, which also bloomed in situ in Gulf deep-waters during the discharge. In contrast, oil addition lacking dispersant stimulated growth of natural hydrocarbon-degrading Marinobacter. Dispersants did not enhance heterotrophic microbial activity or hydrocarbon oxidation rates. Extrapolating this comprehensive data set to real world scenarios questions whether dispersants stimulate microbial oil degradation in deep ocean waters and instead highlights that dispersants can exert a negative effect on microbial hydrocarbon degradation rates.