Effects of PS-MPs on growth, immunity, antioxidant capacity and amino acid metabolism in common carp ( <i>Cyprinus carpio</i> )

To explore the toxic effects of polystyrene microplastics (PS-MPs) on common carp ( Cyprinus carpio ), 0 µg/L, 50 µg/L, 100 µg/L, and 200 µg/L PS-MPs were designated as the control group (CK) and low, medium, and high concentration groups, respectively. Each group had 3 replicates and was exposed to PS-MPs for 15 days. The results showed that, compared with the control group, the weight gain rate and hepatopancreas somatic indices were significantly decreased in 100 and 200 μg/L PS-MPs groups, the visceral index was significantly decreased in 50 and 100 μg/L PS-MPs groups. In comparison to CK, experimental groups (50-200 µg/L PS-MPs) significantly decreased activity levels of acid phosphatase, alkaline phosphatase, and peroxidase while significantly increased activity levels of lysozyme, catalase, and superoxide dismutase. In addition, the malondialdehyde contents of the experimental carp increased significantly in 50 and 100 µg/L PS-MPs groups compared to CK. For hepatic amino acid contents, methionine, threonine, leucine, and isoleucine contents of the hepatopancreas of carp in PS-MPs groups were significantly lower than the control group. Correlation analysis showed that amino acids in the liver closely associated with fish growth, immune, and antioxidant-related indicators. In conclusion, PS-MPs above 50 µg/L in the water body could significantly affect normal growth, weaken the activity of hepatopancreatic immunity-related enzymes and antioxidant capacity, and disturb the balance of amino acid metabolism in common carp. The findings provide valuable insights into optimizing feeding strategies, contribute to more sustainable aquaculture practices, and support the development of policies in aquatic ecosystems.