Our initial studies of the impacts of Deepwater Horizon oil (DHO) have shown that some members of the marine phytoplankton collected from coastal Louisiana waters grow well in seawater containing as much as 100 parts per million wellhead DHO. Control studies show that these assemblages, consisting primarily of chlorophytes, differ markedly from the diatom-dominated community that develops in the absence of DHO. Oil spills therefore can dramatically alter phytoplankton community composition, with likely effects on the production of organic carbon, its transfer to higher trophic levels, and the composition and cycling of dissolved organic matter (DOM).
These observations agree with results of Huang et al., who have reported both stimulation and inhibition of algal growth, with differential responses of phytoplankton to crude oil based on oil concentration, season, and species-specific susceptibility to toxic effects. Others investigators have also documented differential responses to oil at the level of phytoplankton species. Gonzalez et al., for example, showed that responses to oil by diatoms appeared to be size dependent, with larger taxa inhibited and smaller taxa stimulated. This is notable, since smaller taxa experience greater diffusive contact with dissolved hydrocarbons than the larger taxa do.
These and many other studies of oil impacts have been motivated by and explicitly recognized that phytoplankton form the base of aquatic food webs, and that effects of oil on algal populations ultimately become manifest at ecosystem levels. However, while the profound importance of interactions between phytoplankton and bacterioplankton are well known, studies of oil impacts on the former have not incorporated effects on the latter. Likewise, studies of phytoplankton-oil interactions have typically dosed phytoplankton with oil or specific oil hydrocarbons, but then have not followed the hydrocarbons or otherwise characterized the DOM pool, the composition of which is controlled in part by phytoplankton. Understanding oil-phytoplankton-DOM interactions is important, because DOM mediates phytoplankton-bacterioplankton interactions. In addition, direct interactions between bacteria and phytoplankton affect the composition of both communities, while oil directly affects bacterioplankton species composition by stimulating growth of hydrocarbon degraders.
Based on this understanding, work proposed here will involve controlled experiments designed to elucidate (1) the impacts of oil on marine phyto- and bacterioplankton communities,(2) the interactions between these communities and DOM, and (3) effects of changes in phyto- and bacterioplankton communities on oil hydrocarbon degradation.