Changes in chemical composition and copepod toxicity during petroleum photooxidation

Photoproducts can be formed rapidly in the initial phase of a marine oil spill. However, their toxicity is not well understood. In this study, oil was irradiated, chemically characterized, and tested for toxicity in three copepod species (A. tonsa, T. longicornis, C.finmarchicus). Irradiation led to a depletion of polycyclic aromatic hydrocarbons (PAHs) and n-alkanes in oil residues, along with an enrichment in aromatic and aliphatic oil photoproducts. Target lipid model-based calculations of PAH toxic units (TU-PAH) predicted that PAH toxicities were lower in water accommodated fractions (WAFs) of irradiated oil residues (“irradiated WAFs”) than in WAFs of dark-control samples (“dark WAFs”). In contrast, biomimetic extraction (BE) measurements showed increased bioaccumulation potential of irradiated WAFs compared to dark WAFs, mainly driven by photoproducts present in irradiated oil. In line with the BE results, copepod mortality increased in response to irradiated WAFs compared to dark WAFs. Low copepod toxicities were observed for WAFs produced with photooxidized oil slicks collected during the Deepwater Horizon oil spill. The results of this study suggest that while oil photoproducts have the potential to be a significant source of copepod toxicity, the water solubility of these products might mitigate their toxicity at sea.