Mechanism of digestion enzymes and related genes in response to acute ammonia-nitrogen stress in juvenile yellowfin tuna (Thunnus albacares)

To investigate the impact of acute ammonia stress (NH₃-N) on the digestive physiology and gene expression in juvenile yellowfin tuna (Thunnus albacares), we used natural seawater (0 mg/L ammonia) as the control. Ammonia concentrations of 5 and 10 mg/L were set to measure the levels of digestive enzymes in the intestine, liver, stomach, and pyloric cecum at intervals of 6, 24, and 36 h. Additionally, this experiment measured antioxidant and immunity genes in the foregut. A comprehensive evaluation and analysis of these results were then performed. Compared to the control group, an ammonia concentration of 5 mg/L inhibited liver amylase (AMS) and lipase (LPS), while promoting pepsin after 24 h and both AMS and LPS after 36 h in the foregut. The inhibition of LPS activity in the foregut was significantly higher in the 10 mg/L group than in the control group. Moreover, AMS activity in the foregut was remarkably higher than in the control after 6 h, and significantly lower after 24 h. The inhibition of AMS activity in the liver was notably lower than in the control from 6 to 36 h, while liver LPS levels remained similar to the control, and levels of pepsin and pancreatic protease were significantly higher at 6 h. In the 5 mg/L group, compared to the control, the relative expression of the superoxide dismutase 2 (SOD2) gene significantly increased at 6 and 24 h. Catalase (CAT) levels showed a decreasing trend, and glutathione peroxidase 1b (GPX1b) remained similar to the control. In the 10 mg/L group, SOD2 levels markedly increased at 6 h, decreased at 24 h, and CAT levels significantly increased at 36 h, with a marked increase in GPX1b at 24 h. The group treated with ammonia nitrogen exhibited an increasing trend in interleukin 6 receptor (IL-6r) levels, which were comparable to the control from 6 to 36 h. In the 5 mg/L treatment group, interleukin 10 (IL-10) levels were markedly reduced at 24 h but increased by 36 h to levels similar to those of the control group. In the 10 mg/L treatment group, IL-10 levels significantly increased at 36 h. The impact of different ammonia nitrogen levels on the digestibility of juvenile yellowfin tuna was also assessed. Digestibility is enhanced when the ammonia nitrogen level is below 5 mg/L and exposure does not exceed 36 h. Conversely, digestibility is inhibited when the ammonia nitrogen level is between 5 mg/L and 10 mg/L and exposure does not exceed 24 h. This study shows that different concentrations of ammonia nitrogen can affect the digestive activity of juvenile yellowfin tuna. This study provides a scientific foundation for the recirculating water culture of juvenile yellowfin tuna and helps to promote the development of its aquaculture industry.