The effects of sunlight on the composition of exopolymeric substances and subsequent aggregate formation during oil spills

During the Deepwater Horizon oil spill, a large amount of marine oil snow (MOS) was formed in surface waters of the Gulf of Mexico (GOM) that eventually settled to the bottom. MOS consists of a microbially colonized matrix of extracellular polymeric substances (EPS), to which cells, and both ionic (e.g., trace nutrients, Ca2+) and non-ionic (e.g., toxic oil) substances can attach. The secretion of EPS is one of the microbial defense strategies against harmful or stressful environmental situations. In the surface waters, the toxicity of oil can be enhanced by elevated oxidative stress through UV radiation. To test the effects of sunlight on the composition and secretion of EPS and the subsequent aggregation process, we conducted short-term irradiation experiments in three treatments, i.e., control (GOM coastal seawater), water accommodated fraction of oil (WAF), and chemically-enhanced WAF (CEWAF). EPS composition (mainly carbohydrates and proteins) was quantified in the colloidal and aggregate fractions. In addition, bacterial abundance, live/dead cell ratio, particle size distribution, and the ambient hydroxyl radical (OH) formation rate were measured under these conditions. We found that in the presence of oil, natural sunlight stimulated polysaccharide secretion, coinciding with increased reactive oxygen species (ROS; i.e. OH) production. Moreover, formation of larger sized aggregates (>10 μm) was observed in the irradiated WAF treatments. The results support the hypothesis that sunlight plays an important role in MOS formation during an oil spill.