Associations Between Microplastic Burden and Oxidative Stress Biomarkers in the Freshwater Mussel Unio elongatulus

Freshwater ecosystems are increasingly threatened by microplastics (MPs), yet field evidence linking MP burden with physiological responses in freshwater bivalves remains limited. This study investigated the associations between individual MP burden and oxidative stress biomarkers in the freshwater mussel Unio elongatulus collected from the Sacchetti Ditch, northeastern Italy. Mussels were sampled bimonthly over a 10-month monitoring period. The activities of superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase, glutathione S-transferase, glyoxalase I, glyoxalase II, and lactate dehydrogenase, together with metallothionein levels, were determined in the gills and digestive gland. Significant associations with MP burden were detected for glyoxalase II and lactate dehydrogenase in the digestive gland and for glutathione S-transferase in the gills. MP burden was negatively associated with glutathione S-transferase and lactate dehydrogenase activities and positively associated with glyoxalase II activity. Most biomarkers also showed significant variation among sampling periods, indicating substantial temporal variability. Although the observational design does not allow causal relationships to be established, GST, GII, and LDH were the biomarkers most consistently associated with individual MP burden. These biomarkers may therefore represent promising candidates for inclusion in integrated biomonitoring approaches aimed at assessing MP-associated sublethal stress in freshwater ecosystems.