Alteration of growth, hematology, histopathology of tissues and immune-antioxidant genes expression in Nile tilapia following co-exposure of hexavalent chromium and polyamide microplastics

Chromium (Cr) and microplastics (MP) pollution in aquatic environments is increasing, and it is hypothesized that the combined effects of these two pollutants have variable impacts on the physiology of aquatic organisms. An experiment was conducted to evaluate the combined effects of these contaminants on survival, growth, hematology, intestinal and liver histology, oxidative stress, and immunity of Nile tilapia (Oreochromis niloticus). Fish were exposed to T1 (10 mg/L polyamide microplastic; PA-MP), T2 (8 mg/L hexavalent chromium; Cr-VI), T3 (8 mg/L Cr-VI + 10 mg PA-MP) with a control group in triplicates. After 42 days of exposure, it was observed that the ingestion of microplastics in the gastrointestinal tract (GIT), feed conversion ratio (FCR) and mortality were highest in T3 with the co-exposure of Cr-VI with PA-MP. There was an increasing trend in blood glucose concentration and a decreasing trend in hemoglobin concentration observed from T1 to T3. Nuclear and cellular abnormalities became more frequent and histological observation was also aggravated in T3. The relative mRNA expression of antioxidant genes like SOD (superoxide dismutase) and CAT (catalase) was significantly upregulated in T2 and T3 and the immune genes such as TNF-α (Tumor necrosis factor- α), IL-1β (Interleukin-1β) and IFN-γ (Interferon- γ) were also significantly upregulated in T2 and T3 with the exposure of Cr-VI alone and co-exposure of Cr-VI and PA-MP. Therefore, the combined exposure of Cr with PA-MP negatively impacted the overall physiology of Nile tilapia and, there was considerable evidence that polyamide microplastics worsen the overall toxic effects of hexavalent chromium in Nile tilapia.