Chronic ammonia stress caused disruption of intestinal microbiota and damages intestinal structure and function in  yellow catfish (Pelteobagrus fulvidraco)
Ammonia is one of the main oxygen-consuming environmental pollutants in aquatic ecosystems and high concentrations of ammonia are toxic to aquatic organisms. We wanted to know how chronic ammonia stress affects the intestinal microbiota and the intestinal function of yellow catfish (Pelteobagrus fulvidraco). In this study, we explored chronic ammonia stress (total ammonia 0 mg/L, 0.5 mg/L, 2.5 mg/L) on intestinal structure, intestinal function and intestinal microbiota of the yellow catfish. The results of 16 S high-throughput sequencing showed that chronic ammonia stress caused variations of intestinal microbiota. The ACE, Shonnon and Chao indices of gut microbiota showed an increasing trend in the 2.5 mg/L ammonia-stressed group. Ammonia stress also led to changes in the intestinal microbiota and the relative abundance of bacteria. We also found the abundance of family Flavobacteriaceae, genus Vibrio and Pseudomonas (g), species Pseudomonas aeruginosa, Aeromonas veronii (s) and Streptococcus iniae (s) in ammonia nitrogen stress group showed increased tendency. The H&amp;E staining showed that chronic ammonia stress caused in significant congestion and vacuolation of intestinal tissues and reduced significantly in width of intestinal villi. Then, we found chronic ammonia stress significantly decreased in the number of goblet cells by PAS staining. Finally, ammonia stress caused significant decrease in activities of pepsase, chymotrypsin and trypsase. In summary, ammonia stress led to disruption the intestinal microbiota, causing damage of the intestinal structure and affecting the normal physiological function of the intestine.