Effects of Nitrogen-based fertilizers on fish health: Evaluation of stress factors and immune response

Increasing demand for food induces the human population to use excessive fertilizers in agricultural practices. A large quantity of Nitrogen fertilizer is used in agricultural fields compared with phosphate and potassium fertilizers, and unabsorbed excess accumulates in water bodies. Nitrogen-based fertilizers are degraded into ammonia and ammonium ions, which are toxic for fish. The gills and skin of fish are the primary sites of toxic exposure, and through gills, ammonia enters into the fish's bloodstream and reaches different organs and tissues. Ammonia exposure to fish has evidence to damage the fish's red blood cells as well as blood stem cells or hematopoietic stem cells in the liver and head-kidney, which are the major fish organs and showed tissue-specific toxicity. Unionized forms of ammonia can efficiently cross cell membranes and exert more toxicity. Ammonia exposure showed oxidative damage, i.e., lipid peroxidation, protein and DNA denaturation and stress hormone-mediated toxicity, which in turn affect the immune system. Ammonia toxicity disrupts the glutamine metabolic pathway, thereby causing cellular proliferation and death, which is associated with an inflammatory response; mediated through inflammatory cytokines like IL-1, IL-8, and TNF release in exposed fish. Present review deals with such lethal impacts of ammonia on fish health, where we are concerned as the direct consumer of fish and fish products.