Differences in biomarker and behavioral responses to native and chemically dispersed crude and refined fossil oils in zebrafish early life stages

Petroleum products including crude oils and refined distillates are unique environmental pollutants consisting of thousands of compounds with varying physical-chemical properties and resulting toxicity for aquatic biota. Hence, for a reliable risk assessment individual petroleum product toxicity profiles are needed. Furthermore, the influence of oil spill response strategies like the application of chemical dispersants has to be implemented. The present study addressed the toxicity of water-accommodated fractions (WAFs) of two different oil types on fish early life stages on different biological organization levels in the laboratory model species Danio rerio. Experiments with a 3 rd generation dispersant used in loading rated resembling the exposure in experiments with chemically dispersed oils were included, enabling a direct comparability of results. This approach is of high importance as especially the investigation of dispersant toxicity in relevant exposure concentrations is rather scarce. Zebrafish embryos were exposed to different WAFs shortly after and up to 120 hour post fertilization (hpf). Besides phenotypic effects including edema and spine deformations, reduced responses to dark stimuli, increased CYP1A activity and marginal AChE inhibition were observed in sublethal effect concentrations. Both oil types had varying strength of toxicity, which did not correlate with corresponding chemical analysis of target PAHs. Chemically dispersed oils induced stronger acute toxicity in zebrafish embryos compared to native (initial) oil exposure, which was further reflected by very low exposure concentrations for biomarker endpoints. Based on a comparison to the dispersant alone, a higher toxicity of dispersed oils was related to a combination of dispersant toxicity and an elevated crude oil compound bioavailability, due to dispersion-related partitioning kinetics. In contrast to LEWAF and CEWAF neither typical morphological effects nor mechanism-specific toxicity were observed for the dispersant alone, indicating narcosis as the responsible cause of effects.