Impacts of petroleum exposure on DNA integrity and gene expression in the Pacific oyster, Crassostrea gigas

The increase in petroleum use and proposed petroleum expansion projects on the Pacific west coast of British Columbia will increase the risk of oil spills in the marine environment. Advancements in molecular assays have propelled their use in identifying sublethal biomarkers. Molecular biomarkers aid in the early detection of exposure and toxic effects of oil in organisms. Adult Pacific oysters (Crassostrea gigas) were exposed to 3 different oils (marine diesel, crude oil, and diluted bitumen) for 7 d via water accommodated fractions ([WAFs] 1:30 oil/water ratio) diluted at 0 %, 25 %, 50 % and 100 % with seawater. Microsomal CYP1A enzyme activity (EROD) and the mRNA expression of 8 detoxification-related genes (ARNT, CYP2C50, CYP2C23, GST-theta-1, mGST-2, MDR1, CuZnSOD and Hsc-70) were measured in gill tissue at 7 d (exposure), 21 d and 35 d (recovery). DNA damage using the COMET assay in hemocytes was measured under the same exposure and recovery conditions. Individual polycyclic aromatic compound (PAC) concentrations were measured in water (at 0 d, 3 d, 7 d) and in whole soft tissue samples (7 d, 21 d, 35 d) for each oil exposure. The predominant PACs in both water and tissue samples consisted of C1-C4 chain napthalenes, fluorenes, and phenanthrene for all oil types, with crude exhibiting the highest total concentration of PACs in both media. Total PAC concentrations in water samples decreased exponentially from 0 d – 7 d and were rapidly depurated from oyster tissues between 7 d – 21 d. EROD activity was not detected in any oyster tissues, likely due to very low basal enzyme levels and lack of CYP-mediated induction. No significant temporal or concentration-dependent trends were observed in the expression of detoxification-related genes in gills , suggesting that biotransformation is not the primary elimination method for water-soluble PACs in this tissue. Similarly, no significant DNA damage was observed following exposure, suggesting that PACs accumulated did not intercalate with the DNA, or that strand-breaks were not accumulated at a rate that exceeded DNA repair mechanisms.