Effects of ammonia stress on the hemocytes of the Pacific white shrimp Litopenaeus vannamei

Ammonia is an important environmental stress factor in aquaculture. Long-term ammonia stress could affect the normal growth, and also increase the risk for the occurrence of various diseases. In order to learn the mechanism that ammonia caused the outbreak of the shrimp disease, transcriptomics and metabolomics approaches were used to analyze the differential expressions of the genes in hemocytes and different metabolites in the serum of the Pacific white shrimp Litopenaeus vannamei under ammonia exposure. Transcriptional analysis showed that 17 cell apoptosis related genes, seven phagocytosis related genes, 10 immunity related genes and seven cell cycle and lipid metabolism related genes showed differential expressions after ammonia exposure. Metabolomics analysis on the serum showed that 25 differential metabolites were identified in positive and negative ion patterns. They are involved in purine metabolism, amino acids metabolism and lipid metabolism. Injection of two up-regulated metabolites triethanolamine and oxypurinol to normal shrimp could induce apoptosis in normal shrimp. The total hemocytes counts in shrimp showed a significant decrease and the apoptotic cell ratio increased significantly under ammonia exposure. These results suggested that ammonia exposure increased the apoptosis of hemocytes, which affected the immunity of shrimp, and thus caused susceptibility to pathogenic infection. These data will help us understand the mechanism of ammonia stress leading to the immunity decline of shrimp.