The interplay of extracellular polymeric substances and oil/Corexit to affect the petroleum incorporation into sinking marine oil snow in four mesocosms.

Large amounts of oil containing mucous-like marine snow formed in surface waters adjacent to the Deepwater Horizon spill that was implicated in oil delivery to the seafloor. However, whether chemical dispersants that were used increased or decreased the oil incorporation and sedimentation efficiency, and how exopolymeric substances (EPS) are involved in this process remains unresolved. To investigate the microbial responses to oil and dispersants in different oceanic settings, indicated by EPS production, petro- and non-petro carbon sedimentation, four mesocosm (M) experiments were conducted: 1) nearshore seawater with a natural microbial consortia (M2); 2) offshore seawater with f/20 nutrients (M3); 3) coastal seawater with f/20 nutrients (M4); 4) nearshore seawater with a natural microbial consortia for a longer duration (M5). Four treatments were conducted in M2, M3 and M4 whereas only three in M5: 1) a water accommodated fraction of oil (WAF), 2) a chemically-enhanced WAF prepared with Corexit (CEWAF, not in M5), 3) a 10-fold diluted CEWAF (DCEWAF); and 4) controls. Overall, oil and dispersants input, nutrient and microbial biomass addition enhanced EPS production. Dispersant addition tended to induce the production of EPS with higher protein/carbohydrate (P/C) ratios, irrespective of oceanic regions. EPS produced in M4 was generally more hydrophobic than that produced in M3. The P/C ratio of EPS in both the aggregate and the colloidal fraction was a key factor that regulated oil contribution to sinking aggregates, based on the close correlation with %petro-carbon in these fractions. In the short term (4-5 days), both the petro and non-petro carbon sedimentation efficiencies showed decreasing trends when oil/dispersants were present. In comparison, in the longer-term (16 days), petro-carbon sedimentation efficiency was less influenced by dispersants, possibly due to biological and physicochemical changes of the components of the oil-EPS-mineral phase system, which cooperatively controlled the sinking velocities of the aggregates.