Climate change meets emerging contaminants: salinity shapes PFAS toxicity in Mytilus galloprovincialis

Climate change is leading to more frequent and extreme salinity fluctuations in marine environments, and the presence of emerging per- and polyfluoroalkyl contaminants (PFAS) is attracting increasing attention. These two stressors can interact and produce different toxic scenarios. The present study investigated the combined effects of salinity (22 and 32) and perfluorohexyl acetic acid (6:2 FTA) at 10 ng/L on the biochemical responses of the Mediterranean mussel Mytilus galloprovincialis. After 28 days of exposure, a set of biochemical parameters was assessed: total antioxidant capacity (TAC), superoxide dismutase (SOD), glutathione peroxidase (GPx), glutathione reductase (GR), lipid peroxidation (LPO), protein carbonylation (PC), proline (PROL), succinate dehydrogenase system activity (MTT), electron transport system (ETS), protein (PROT), lipid (LIP), acetylcholinesterase (AChE), reduced glutathione (GSH) and oxidized glutathione (GSSG). Results revealed that salinity was the primary driver of biochemical variation. Mussels at salinity 22 showed reduced TAC and higher SOD activity, reflecting increased oxidative stress associated with the osmotic challenge. Salinity-dependent effects also emerged in response to 6:2 FTA exposure: mussels at salinity 22 exhibited lower LPO levels and MTT activity, suggesting moderate metabolic alteration. The AChE activity was higher at salinity 32, indicating an altered neurophysiological response under conditions closer to the species' optimal salinity range. Overall, exposure to 6:2 FTA resulted in limited effects on the measured biochemical endpoints under the specific experimental conditions tested in M. galloprovincialis. These findings should therefore be interpreted within the constraints of the experimental design, particularly the use of a single nominal concentration and exposure duration, but further highlight the importance of incorporating environmental variability into ecotoxicological assessments, especially in the context of climate change.