Protein: Polysaccharide ratio in exopolymeric substances controlling the surface tension of seawater in the presence or absence of surrogate Macondo oil with and without Corexit

Amphiphilic substances can interact with oil in the water to form emulsions and aggregates; dispersants are designed to form emulsions to prevent oil droplets from coalescing and stabilize them in a suspension. Amphiphilic extracellular polymeric substances (EPS) concentrated at the air-water interface where surface tension was measured to determine their propensity for formation of emulsions and/or aggregates. Here we investigated mechanisms governing the self-assembly and phase separation for protein-polysaccharide-oil-dispersant interactions through measurements of surface tension, chemical composition, and confocal microscopy. EPS colloid fractions from treatments of WAF (water accommodated fraction of oil), CEWAF (chemically enhanced WAF using the dispersant Corexit), and a control mesocosm were examined. In analyzing the size fractions of water column samples, it was found that treatments with oil and/or Corexit showed EPS with enhanced protein:polysaccharide carbon-based ratios and lower surface tension (SFT), suggesting the effective bioemulsifying effects of proteins. In addition, EPS model constituents of protein slightly increased SFT at low concentrations of less than a few mg/L, but decreased it at higher concentrations of 8 mg/L or more. These model molecules appear to be more efficient than Corexit in inducing the self-assembly of micelles in the seawater even when only very low concentrations of these constituents are present. Our results suggest that EPS are more efficient than Corexit at forming micelles. Results from this study provides mechanistic insights into the fate and distribution of oil in the surface ocean.