Combined effect of mercury and ammonia toxicity and its mitigation through selenium nanoparticles in fish

An experiment was conducted to mitigate mercury and ammonia toxicity (Hg + NH₃) in Oreochromis niloticus (GIFT strain) using selenium nanoparticles (Se-NPs). The Se-NPs were synthesized using green methods, employing fish waste as the substrate. Experimental diets were prepared by supplementing Se-NPs at 0, 0.3, and 0.6 mg kg⁻¹. The oxidative stress enzymes, including catalase (CAT) and glutathione peroxidase (GPx), in the liver and kidney tissues were significantly reduced by Se-NPs at 0.3 and 0.6 mg kg⁻¹ under Hg + NH₃ stress compared to the control and stressor in 20 and 40 day periods. Additionally, superoxide dismutase (SOD) activity in the kidney at 20 days and in the liver at 40 days was significantly reduced by supplementation of Se-NPs under similar conditions. The activity of acetylcholine esterase (AChE), was significantly inhibited by Hg + NH₃ toxicity. Whereas, AChE activity was enhanced by Se-NPs supplementation at 0.3 and 0.6 mg kg⁻¹ during 20 and 40 day intervals. The gene expression of HSP70, iNOS, CYP450, Caspase-3a, and TNFα in liver tissue, and MYST in muscle tissue was upregulated by Hg+NH₃ toxicity. However, this upregulation was significantly downregulated by supplementation of Se-NPs at 0.3 and 0.6 mg kg⁻¹ under Hg + NH₃ stress. Moreover, immunoglobulin (Ig) and growth hormone (GH) levels were noticeably upregulated with Se-NPs compared to the control and Hg+NH₃ stress. The activities of alanine aminotransferase (ALT), aspartate aminotransferase (AST), lactate dehydrogenase (LDH), and malate dehydrogenase (MDH) in liver and gill tissues, were significantly elevated by Hg+NH₃ toxicity, were reduced by Se-NPs diet. Conversely, digestive enzyme activities, including protease, amylase, and lipase, were significantly enhanced by Se-NPs under stress conditions. Dietary supplementation with Se-NPs at 0.3 and 0.6 mg kg⁻¹ improved growth performance parameters such as final weight gain percentage, feed conversion ratio, protein conversion ratio, specific growth rate, daily growth index, and relative feed intake compared to the control and other groups. DNA damage, assessed in terms of tail DNA percentage, was significantly reduced with Se-NPs supplementation. Additionally, mercury detoxification was significantly enhanced with Se-NPs-supplemented diets. In conclusion, this study demonstrates that dietary Se-NPs effectively alleviate the adverse effects of mercury and ammonia toxicity by modulating antioxidant status, enhancing immunomodulation, and mitigating stress biomarker impacts through changes in gene expression in fish.