The copepod Eurytemora affinis as a relevant species to assess estuarine sediment toxicity by combining sub-individual and individual endpoints: effects on gene expression and swimming behavior
Compared to freshwater ecosystems, the health status of estuarine waters remains little studied despite their importance for many species. They also represent a zone of interest for Human settlements that make them the final sink of pollution in both the water column and sediment. Once in sediments, pollutants could represent a threat to benthic as well as pelagic estuarine species through resuspension events. In the Seine estuary, the copepod Eurytemora affinis has been previously presented as a relevant species to assess resuspended sediment contamination through the use of fitness-related effects at the individual level. The aim of the present study was to use E. affinis copepods to assess estuarine sediment-derived elutriates toxicity using both a molecular (i.e. transcriptomics) and a behavioral approach. Two sites along the Seine estuary were sampled. They were both under anthropic pressures from the industrial-port activities or wastewater treatment plants (i.e. Tancarville) or agricultural pressure from freshwater affluent (i.e. Fatouville). The analysis of sediments used to prepare elutriates reveals that both sites have close contamination profiles. The transcriptomic analysis reveals that exposure to both sites triggers the dysregulation of genes involved in biological function as defense response, immunity, ecdysone pathway or neurotoxicity. This analysis also reveals a higher count of dysregulated genes in the Fatouville site compared to the Tancarville despite their close contamination profile. These results emphasize the sensitivity of this molecular approach to assess environmental matrix toxicity with E. affinis. The analysis of the swimming behavior of E. affinis did not highlight significant effects after both sites elutriate exposure. However, our strategy to assess E. affinis swimming behavior (i.e the combination of the DanioVision observation chamber and the EthoVision analysis software) allows the discrimination of basal swimming behavior in this species. Thus, it represents a promising standardized tool to assess copepods swimming behavior in ecotoxicological studies. Overall design: Adult copepods (Eurytemora affinis) sampled from the Seine estuary were exposed to sediment-derived elutriates (2500 mg/L and 350 mg/L) sampled from two sites (i.e. Tancarville and Fatouville). Exposures were performed in biological triplicates for each conditions (Control, Tancarville 350 mg/L, Tancarville 2500 mg/L, Fatouville 350 mg/L and Fatouville mg/L). After 72 h of exposure, pools of 30 males per biological triplicates were sorted, placed in microtubes with ceramic beads (CK14, Bertin, France), washed with RNA-free water, quickly dry frozen in liquid nitrogen and stored at -80 °C before the RNA extraction.