The present study aimed at assessing the singular and combined effects of water copper and polyvinyl chloride microplastic (MPVC) on intestinal copper accumulation, histopathological damage, and stress‐/immune‐related genes' expression in common carp, Cyprinus carpio. Four groups of fish were maintained in triplicate: control (kept in clean water), Cu (exposed to 0.25 mg/L of copper), MPVC (exposed to 0.5 mg/L of MPVC), and Cu‐MPVC (exposed to 0.25 mg/L of copper + 0.5 mg/L of MPVC). After 14‐day exposure, the fish of Cu and Cu‐MPVC treatments exhibited significantly higher intestinal copper contents, compared to the fish of control and MPVC treatments. In this regard, the Cu‐MPVC fish had significantly higher copper content than the Cu fish. Exposure to copper and/or MPVC significantly upregulated the intestinal heat shock protein 70 (hsp70), cytochrome P450 family 1 subfamily A member 1 (cyp1a1), lysozyme (lys), defensin (def), mucin 2 (muc2), and mucin 5 (muc5) expression. The highest expression of hsp70, cyp1a1, lys, and def was related to Cu‐MPVC treatment; whereas, the highest expression of muc2 and muc5 was observed in Cu and MPVC treatments. Exposure to copper and/or MPVC induced intestinal damage, which Cu‐MPVC fish exhibited the highest severity. The present study revealed that exposure to copper and/or MPVC causes intestinal histopathological damage and upregulation in stress‐ and immune‐related genes' expression. The most serious effects were observed in Cu‐MPVC treatment that might be due to additive effects of copper and MPVC and/or higher copper accumulation in this treatment.

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The present trial investigates the effects of soil-derived Streptomyces chartreusis KU324443 as a probiotic on growth performance, mucosal and serum immune parameters, and immune and antioxidant-associated gene expression in common carp (Cyprinus carpio). In a two-month experiment, carps were fed with varying levels of S. chartreusis KU324443 (0 (control), 105 (S1), 106 (S2), and 107 (S3) CFU/g). Then, performance, skin mucus, and serum immune parameters besides immune and antioxidant-related gene expression (IL1 and Lyz, CAT and SOD) were measured. Fish fed S. chartreusis-supplemented diet showed a significant increase in growth performance parameters ( P < 0.05 ) regardless of inclusion levels. Including different doses of S. chartreusis significantly increased serum total Ig and lysozyme activity compared to those fed the control diet ( P < 0.05 ). While no significant difference was noticed in the case of skin mucus total Ig ( P > 0.05 ), lysozyme activity showed a significant increase in S. chartreusis-treated carps ( P < 0.05 ). No significant differences ( P > 0.05 ) were noticed in the case of serum antioxidant enzyme activity (CAT, SOD, and GPx) of S. chartreusis-fed carps and control. However, fish fed the control diet recorded the highest SOD and GPx enzyme activity in skin mucus of common carp, while no significant differences were noticed in the case of mucus CAT enzyme activity in common carp ( P > 0.05 ). Gene expression studies revealed noticeable alterations between treated fish and control. Fish in S3 treatment had significantly higher CAT, SOD, Lyz, and IL1 expression ( P < 0.05 ). These results confirmed the beneficial effects of a soil-derived probiotic (S. chartreusis) on the performance and health of common carp.

Common carp, Cyprinus carpio, is one of the most important aquaculture species in the world. About 4.6 million tones common carp are annually produced, accounting for 8% of the world's total finfish production (FAO, 2020). Increased global demand for edible fish has led to progressive increase in fish rearing density to increase yield and land-use efficiency; however, such a rearing intensification has