The Impact of Acute Ammonia Nitrogen Stress on Serum Biochemical Indicators and Spleen Gene Expression in Juvenile Yellowfin Tuna (Thunnus albacares)

The presence of ammonia nitrogen in water has a significant impact on the serum and spleen of fish, potentially leading to changes in substances such as proteins in the serum while also causing damage to the immune function of the spleen. To investigate the effects of ammonia nitrogen (NH₃-N) stress on juvenile yellowfin tuna (<i>Thunnus albacares</i>), this study established three NH₃-N concentrations, 0, 5, and 10 mg/L, denoted as L0, L1, and L2, respectively. Serum and spleen samples were collected at 6, 24, and 36 h. The effects of different NH₃-N concentrations and exposure times on the physiological status of juvenile fish were investigated by analyzing serum biochemical indices and splenic gene expression. The results indicate that in the L1 group, the serum high-density lipoprotein cholesterol (HDL-C), triglycerides (TG), complement 3 (C3) and complement 4 (C4) levels, and acid phosphatase (ACP) activity showed a trend of initially increasing and then decreasing. In the L2 group, the serum low-density lipoprotein cholesterol (LDL-C), HDL-C, and C4 levels and ACP activity also displayed an initially rising and then declining trend, while TG, C3, and creatinine (CRE) levels and alkaline phosphatase (AKP) activity showed an upward trend. In the L1 group, glutathione peroxidase 1b (<i>GPX1b</i>), interleukin 10 (<i>IL-10</i>), interleukin 6 receptor (<i>IL-6r</i>), and tumor necrosis factor alpha (<i>TNF-α</i>) gene expression levels in the spleen exhibited a trend of first increasing and then decreasing. In the L2 group, <i>IL-10</i>, <i>IL-6r</i>, tumor necrosis factor beta (<i>TNF-β</i>), caspase 2 (<i>casp2</i>), and caspase 9 (<i>casp9</i>) gene expression levels in the spleen also showed an initial increase followed by a decrease. When NH₃-N levels are below 5 mg/L, it is recommended to limit stress exposure to no more than 36 h for the juvenile fish. For concentrations ranging from 5 to 10 mg/L, stress should be strictly controlled to within 24 h. Exposure to high NH₃-N levels may affect biochemical indicators such as serum lipid metabolism, immunity, and metabolism in juvenile fish, and may damage the expression of antioxidant, immune gene, and apoptosis factors in the spleen. This study aims to deepen our understanding of the effects of NH₃-N on juvenile tuna, with the goal of establishing effective water quality monitoring and management strategies. This will ensure the quality of aquaculture water, reduce the harm caused by NH₃-N to juvenile yellowfin tuna, and enhance aquaculture efficiency and product quality.