Effects of hyperosmotic stress on the intestinal microbiota, transcriptome, and immune function of mandarin fish (Siniperca chuatsi)

As a specific ecological factor of the aquatic environment, salinity can significantly affect intestinal and intestinal microbial homeostasis, which play essential roles in maintaining host health and antibacterial function. To evaluate the effects of saline-alkali water on host health and immune response, in this study, the effects of hyperosmotic stress on intestinal functions, including the intestinal microecosystem, intestinal immune response-related gene expression and intestinal antioxidant capacity, were investigated in mandarin fish (Siniperca chuatsi) after 45 days of exposure to freshwater (0 ppt salinity) or 3 ppt salinity, 6 ppt salinity, 9 ppt salinity, and 12 ppt salinity water environments. The results revealed that the intestinal microbial community composition could be altered and reshaped by hyperosmotic stress. Notably, the relative abundances of the probiotic bacterium Lactobacillus, the pathogenic bacterium Vibrio, and the metabolism-related bacteria Cetobacterium and Gluconobacter in the 6 ppt salinity group was higher than those in other groups, whereas no significant difference was observed among five groups. The analysis of intestinal transcriptome revealed several differentially expressed genes (DEGs) associated with several immune-related DEGs related to proinflammatory cytokines, anti-inflammatory cytokines, antioxidant defense, antimicrobial effects, and cell apoptosis. According to an analysis of intestinal physiological properties, the activity of alkaline phosphatase (AKP) and acid phosphatase (ACP) was significantly increased by hyperosmotic stress, especially in the 6 ppt salinity group. The present study provides essential insights into the influences of saline-alkali water on the health status and immune functions of mandarin fish; additionally, it provides an important foundation for further studies on the effects of complex relationship between intestinal flora and complex host environment on host disease resistance.