Abstract

On April 20, 2010, the explosion of the Deepwater Horizon Platform began releasing crude oil into the Gulf of Mexico. BP reportedly released 1.84 million gallons of the chemical dispersant Corexit® 9500 to help control the spill. The direct toxicity of Corexit® on marine organisms is not well studied. The objectives of this study were to evaluate the toxicity of Corexit® in Eastern oysters (Crassostrea virginica) following acute exposure. Experimental exposures were performed in 20-gallon tanks using artificial seawater spiked once with Corexit®. Oyster tissue and water samples were collected following exposure and analyzed using ultra-performance liquid chromatography-tandem mass spectroscopy. In addition, clearance rates were measured as the ability of oysters to feed on the phytoplankton Tetraselmis. Hemolymph was collected from the adductor muscle for the evaluation of two innate immune functions, the ability of hemocytes (granulocytes and hyalinocytes) to engulf fluorescent beads (phagocytosis), and the ability of hemocytes to produce H₂O₂ (respiratory burst), using flow cytometry. While concentrations as high as 500 ppm did not result in significant mortality after 4 days (i.e., the 96-hour LC₅₀ was >500ppm), the seven-day LC₅₀ was 50 ppm. Following a 1-day exposure, tissue concentrations revealed that oysters could quickly uptake Corexit® in the first 24 hours of exposure, with tissue concentrations of 11 and 23 ppm following exposure to water concentrations of 10 and 20 ppm in water, respectively. However, higher water concentrations (>20 ppm) did not always result in proportionally higher tissue concentrations. Exposure of oysters to 10 ppm Corexit® for 1 hour significantly reduced clearance rates by approximately 60% compared to control, while higher concentrations appeared to prevent any oyster feeding, possibly explaining the plateau in tissue concentrations. No significant changes were found for phagocytosis after 1 or 3 days of exposure, but the respiratory burst was significantly reduced at 20, 30, 40, and 50 ppm following a 3, but not a 1-day exposure. The inhibition of respiratory burst may prevent the destruction of internalized pathogens, thereby increasing the risk of oyster susceptibility to disease. This continuing study will assess immune functions, clearance rates, tissue concentrations and histopathological changes following exposure to sweet Louisiana crude oil and a Corexit®/oil mixture following 1, 3 and 7 days of exposure. Comparative data on the relative toxic effects of oil dispersants and crude oil will be instrumental in making scientifically sound and informed decisions regarding dispersant use in future oil spills.

Acknowledgements

This study was partially funded by the Morris Animal Foundation (non-lethal experiments).