Partitioning of oil compounds into marine oil snow: Insights into prevailing mechanisms and dispersant effects

The sedimentation of oil via marine snow was observed during and after the Deepwater Horizon oil spill in 2010. We conducted laboratory experiments to characterize the chemical composition of oil associated with marine oil snow (MOS), identify mechanisms responsible for the incorporation of oil compounds into MOS, and describe the impact of the dispersant Corexit on oil incorporation into MOS. Phytoplankton was incubated in roller tanks in the presence or absence of Macondo crude oil and Corexit and the partitioning of n-alkanes and PAHs between MOS > 1 mm, the dissolved phase and particles < 1 mm was analyzed. Oil incorporation into MOS depended largely on the physiochemical properties of the respective oil compounds. Insoluble compounds, namely n-alkanes and high molecular weight (HMW) PAHs, were integrated into MOS within entire oil droplets that were scavenged by phytoplankton. The water-soluble fraction of the oil, namely low molecular weight PAHs, was sorbed by cells in MOS. Droplet incorporation took place immediately, while sorption only occurred after an experimental period longer than 1 day. The dispersant Corexit significantly increased the amount of oil trapped in MOS, and caused MOS to be enriched in HMW oil compounds, of which many act as toxins. Alteration of the chemical composition of oil in MOS after an experimental period of 4 days suggests biodegradation of oil compounds, especially in the presence of Corexit. These results allow further insights into processes affecting MOS formation and sedimentation and assist future research and spill response planning.