Acute Total Ammonia Nitrogen (TAN) stress affects the skin metabolism, antioxidant capacity, immune function, and tissue structure of juvenile yellowfin tuna (Thunnus albacares).

This study investigated the effects of acute ammonia-nitrogen stress on the skin immune function, metabolism, and antioxidant capacity of juveniles yellowfin tuna (Thunnus albacares). The experiment included three treatment groups with ammonia-nitrogen concentrations of 0 (control group), 5, and 10 mg/L (All ammonia nitrogen concentrations are expressed as nitrogen, with units in mg/L). with samples taken at 6, 24, and 36 h to evaluate skin enzyme activity, gene expression, and tissue structure. Elevated metabolic responses of ALT and AST enzymes were observed following ammonia nitrogen exposure. The MDA content increased at6 hours of stress, while the activities of the antioxidant enzymes CAT and GSH-Px correspondingly rose, indicating that oxidative stress had already intensified significantly at the onset of stress. As the stress duration extended to 24 h, both CAT and GSH-Px activities showed a declining trend, suggesting that the organism's antioxidant capacity gradually became dysregulated under continuous stress. Concurrently, increased activity of immune-associated enzymes (AKP and ACP) and marked alterations in TNF-α and TNF-β gene expression implied heightened lysosomal activity to eliminate damaged cells. These responses induced inflammatory cascades in cutaneous tissues to some extent. Histological analysis revealed marked epidermal thickening, cellular edema and necrosis, along with abnormal changes in the numbers of mucous cells and pigment cells. The above results indicate that ammonia nitrogen stress impairs skin integrity through mechanisms involving oxidative stress, metabolic dysregulation, and immune dysfunction. The research advances both fundamental knowledge of ammonia toxicity and applied approaches for better aquaculture management.