Tissue concentration, mortality, and response of subcellular biomarkers in Dreissena polymorpha exposed to nickel: absence of dose-response relationships for biomarkers despite dose-dependent toxicity

Biomarkers are biological responses that inform about stress or toxic effects on sentinel species and they are widely used in environmental monitoring. However, interpreting many biomarkers remains a challenge at present, before we can move towards codifying their responses in terms of environmental quality. The aim of our study was to bring a better perspective to this scientific issue by defining threshold values based on subcellular biochemical response, that would enable to conclude on the health of the entire organism. To define such threshold values, we tried to establish dose-response relationships between biomarker responses and increasing concentrations of a well-known contaminant. Dreissena polymorpha was exposed to between 6 and 20 concentrations of nickel in order to identify potential dose-response relationships. A set of 10 biomarkers representative of antioxidant and antitoxic defences (SOD, GPx, GST, TAC), energetic metabolism (ETS, LDH) and cellular damage (LOOH, CSP3) were measured, as well as internal nickel concentrations. The experiment was conducted at three different periods in May 2023, in July 2023 and in January 2024 to consider the seasonal effect and it was performed with mussels originating from two different populations to see if the results were repeatable. Surprisingly, whatever the season and whatever the population, no dose-response relationship was found for the biomarkers measured, despite a dose-dependent increase in mortality and tissue concentrations. There was a high inter-individual variability both in control and exposure treatments, making it impossible to decipher a specific response pattern. These results did not enable threshold values to be defined as initially planned, since it seems that each mussel finds its own way of dealing with contaminant-induced stress.