Does time matter? Organ-specific stress responses to cobalt in Mytilus galloprovincialis across salinities

Cobalt (Co) pollution in aquatic ecosystems is rising due to its increased use in battery production. This study examines organ-specific biochemical responses of the mussel Mytilus galloprovincialis to Co exposure under three different salinities (20, 30, and 40) over 14 and 28 days. Mussels were exposed to 100 μg/L Co, and biomarkers of metabolism, oxidative stress, detoxification, and neurotoxicity were analyzed in the digestive gland, gills, mantle, and muscle. Exposure duration had a stronger influence on biochemical responses than salinity or Co contamination. A time-dependent response was evident, particularly in the digestive gland and gills. Higher Co accumulation was observed at salinity 20, suggesting that ionic competition affects metal uptake. After 28 days, superoxide dismutase and glutathione peroxidase activities increased, while total antioxidant capacity decreased, indicating oxidative stress. Biotransformation enzyme activities peaked at 14 days but were later inhibited, suggesting an early failure in detoxification. The gills were strongly associated with oxidative stress markers, while the digestive gland played a key role in detoxification. Acetylcholinesterase activity was consistently higher in the gills, indicating an enhanced neurotoxic response. These findings demonstrate that exposure duration is a primary determinant of cobalt-induced biochemical alterations in marine bivalves and should be explicitly integrated into risk assessment frameworks, particularly under variable salinity conditions that modulate metal bioavailability and toxicity.