Physiological and transcriptomic analysis of the tetraploid Pacific oyster (Crassostrea gigas) under acute ammonia nitrogen stress

In recent years, high-density oyster farming has led to the deterioration of the environment in the cultured sea area, especially in the high-temperature summer, the decomposition and death of organic organisms and their excreta will lead to the increase of ammonia nitrogen content. Ammonia nitrogen has become one of the major environmental pollutants affecting the normal physiological activities of oysters. The tetraploid Crassostrea gigas is a key germplasm for the production of triploid C. gigas, however, the molecular mechanism of ammonia nitrogen action in its body is not clear. The study showed that within 48 h of exposure to 10 mg/L ammonia nitrogen, the activities of catalase (CAT) and glutathione peroxidase (GSH-Px) in the hepatopancreas increased and then decreased, the activity of superoxide dismutase (SOD) continuously increased, no significant change in malondialdehyde (MDA) content was observed, and tissue damage in the hepatopancreas gradually aggravated. In addition, a total of 2375 differentially expressed genes (DEGs) were identified after ammonia nitrogen exposure. Significantly enriched pathways for most DEGs included FOXO signaling pathway, apoptosis, protein processing in endoplasmic reticulum, and glutathione metabolism pathway, critical for antioxidant defense, protein homeostasis, and apoptotic regulation. Overall, ammonia nitrogen exposure affected the oxidative-antioxidant balance and endoplasmic reticulum homeostatic balance, which ultimately led to apoptosis and tissue damage. These findings will facilitate research into the molecular mechanisms of oyster ammonia nitrogen stress response and aid molecular breeding of ammonia-tolerant C. gigas.