Transcriptomic and physiological analyses reveal the response mechanism of the liver under acute ammonia stress in the Chinese soft-shelled turtle (Pelodiscus sinensis)

Greenhouse intensive aquaculture is a common culture mode for juvenile Chinese soft-shelled turtles (CSST). Under this culture mode, the juvenile CSST are vulnerable to the stress of high ammonia‑nitrogen levels in the water, resulting in a suboptimal health state that may lead to disease susceptibility. However, few studies have reported the response mechanism of the CSST in the ammonia‑nitrogen environment. In this study, we detected the lethal range concentration of non-ionic ammonia‑nitrogen in CSST and examined the impact of acute ammonia nitrogen exposure for durations of 0 h, 12 h, 24 h, 48 h, 96 h, and 144 h on the blood indices and liver function in CSST. The results show that the lethal concentration range of non-ionic ammonia‑nitrogen is 23.0 mg/mL to 70.5 mg/mL. In the blood, blood ammonia, urea nitrogen and aspartate aminotransferase activity increased significantly, suggesting that the high ammonia‑nitrogen environment caused hyperammonemia, increased urea synthesis and liver damage in the CSST. The impact of acute ammonia nitrogen on the liver function of the CSST was assessed through analysis of liver histopathology, antioxidant defense, ammonia metabolism pathway, and transcriptome data. The results showed that the hepatic cord structure within the liver tissue of the CSST was destroyed, with the presence of vacuoles and edema. In addition, hepatic superoxide dismutase (SOD) and catalase (CAT) enzyme activities and Malondialdehyde (MDA) content were significantly increased, and the mRNA expression levels of Mn-SOD, Cu/Zn-SOD, and CAT were significantly increased, suggesting that the hepatic antioxidant system was activated to resist the environmental stresses caused by ammonia‑nitrogen. Moreover, there was a significant increase in the enzyme activity and expression levels of glutamate dehydrogenase (GDH), and a significant decrease in the enzyme activity and no significant change in the expression levels of glutamine synthetase (GS), suggesting that NH4+ is synthesized from glutamate by GDH in the liver to enter the urea metabolic cycle, and that glutamine is not synthesized for storage. Analysis of transcriptome data revealed potential effects on energy metabolism and lipid metabolism in response to short-term (48 h) ammonia‑nitrogen stress. In contrast, prolonged (96 h) exposure to ammonia nitrogen stress may increase the susceptibility of CSST to pathogens, and trigger the tissue repair mechanisms in the liver. These findings provide n