Size-Dependent Tissue Translocation and Physiological Responses to Dietary Polystyrene Microplastics in Salmo trutta

Microplastics (MPs) are prevalent in freshwater systems; consequently, fish ingest them either accidentally or intentionally. Once ingested, MPs can translocate to various organs and cause physiological effects. Most studies have focused on tropical and marine fishes, and many have used mass-based methods that measure exposure only by the total mass of microplastics, ignoring particle number and size. These studies have also rarely examined MP effects or fate after a depuration period, limiting our understanding of MP impacts on temperate fishes, hindering the harmonisation of toxicological studies, and complicating assessments of food safety for cultured and wild fish. This study investigated the physiological impacts of dietary exposure to polystyrene microplastics (PS-MPs; 1–10 µm) in Salmo trutta fed a diet with ~5.4 × 106 PS-MPs g−1 feed for 21 days, followed by a 90-day depuration period. PS-MPs translocation from the intestine to the liver and muscle was investigated. Enzymatic biomarkers of oxidative stress and metabolism were analysed in the liver, digestive enzyme activity was assessed in the intestine, and inflammatory enzyme responses were evaluated in both liver and intestinal tissues. In addition, malondialdehyde (MDA) concentration, an indicator of lipid peroxidation, was quantified in blood, muscle, and liver samples. Results show that 1–5 µm PS-MPs translocated to the liver and muscle, while 10 µm particles largely remained in the intestine, with a small fraction detected in muscle tissue but not in the liver. Most biochemical markers were unaffected; however, both trypsin and peroxidase activities significantly decreased after 21 days, and lipid peroxidation increased in blood following 90 days of depuration. PS-MPs persisted in muscle following 90 days of depuration. These findings demonstrate that dietary exposure to PS-MPs in the size range 1–10 µm leads to selective physiological alterations in S. trutta and results in persistent accumulation of MPs in organs, especially muscle tissue consumed by humans, highlighting a clear concern for food safety.