Immunomodulation in Eastern Oysters Following Exposure to Polycyclic Aromatic Hydrocarbons

Polycyclic Aromatic Hydrocarbons (PAHs) are a group of persistent organic pollutants that have been shown to alter the health of the eastern oyster. The presence of these compounds in shellfish-harvesting areas has stimulated the need to understand the effects of PAHs on the physiology of bivalve species. In this study, the effects of microalgal-food-borne PAHs on oyster hemocyte immune-defense functions were examined. Flow-cytometric techniques were used to measure hemocyte characteristics (e.g. hemocyte types and viability) and functions (e.g. adhesion, phagocytosis, and generation of reactive oxygen species) in laboratory and field studies. In the laboratory, a microphytobenthic diatom, Nitzschia brevirostris, was shown to accumulate PAH compounds and pass these compounds on to feeding oysters. Increases in circulating hemocyte densities and reactive-oxygen species (ROS) production were exhibited in the exposed oysters. Similarly, direct exposure of PAH compounds to oyster hemocytes in vitro resulted in a reduction in granular hemocyte densities, decreased adhesion of hemocytes, and lowered ROS production. Contrasting immune profiles of natural oyster populations located in a relatively pristine site and a contaminated site, from two Florida bays, were identified. In a follow-up field study, oysters transplanted from a relatively pristine site to a contaminated site developed hemocyte characteristics (e.g. increased hemocyte densities, elevated ROS production, and lower phagocytic activity) similar to those observed in PAH-exposed oysters in laboratory studies. Results from both laboratory and field studies indicate that PAHs in the environment can be responsible for immunomodulation in oysters.