Bioaccumulation and biochemical responses to perfluorooctane sulfonate (PFOS) exposure in the freshwater benthic leech (Poecilobdella manillensis).

Perfluorooctane sulfonate (PFOS) is a persistent per- and polyfluoroalkyl substance (PFASs) frequently detected in aquatic environments, where its accumulation in sediment-associated organisms raises increasing ecological concern. While PFOS toxicity has been extensively investigated in fish and other conventional model organisms, limited information is available regarding its bioaccumulation behavior and biochemical responses in freshwater benthic annelids. In this study, the medicinal leech Poecilobdella manillensis was used as a sediment-associated model organism to evaluate PFOS accumulation dynamics and associated biochemical responses under 14-day exposure conditions. PFOS bioaccumulation, antioxidant-related biomarkers, and antithrombin activity were assessed at four exposure concentrations using a static sediment-water microcosm system. Toxic kinetic parameters, including absorption rate constant (K1) and release rate constant (K2), were estimated using a dual-box dynamic model. During exposure, PFOS induced significant oxidative stress in P. manillensis (P < 0.05). Total superoxide dismutase (T-SOD) activity initially decreased and subsequently increased, whereas total antioxidant capacity (T-AOC) showed an inverted U-shaped temporal response. Antithrombin activity exhibited significant temporal changes and was significantly correlated with total protein (TP), T-SOD, and T-AOC (P < 0.05), indicating associations between antithrombin activity and biochemical responses under contaminant stress. PFOS accumulation increased with exposure duration, particularly under high PFOS exposure, with K1 consistently exceeding K2, suggesting continued PFOS uptake during the exposure period. These findings provide preliminary evidence for the potential use of medicinal leeches as freshwater benthic biomonitoring organisms and contribute to understanding the ecological risks of PFOS in sediment-associated aquatic environments.