Effects of ammonia exposure on oxidative stress, immune enzyme activities, and intestinal microbiota of Pacific white shrimp Litopenaeus vannamei

An ammonia stress experiment was conducted by adjusting the concentration of total ammonia nitrogen (TAN) to 20 mg/L using ammonium chloride. The shrimp without ammonia stress was set as the control group. A total of 300 shrimp were randomly divided into six circular fiberglass tanks, each tank containing 50 shrimp. Antioxidant and immunological enzyme activities were investigated at five time points: 0 h, 12 h, 24 h, 48 h, and 96 h. The shrimp sampled at the initial time point were set as the control group. The samples used for intestinal microbiota determination were taken at two time points: 0 h and 96 h. All of the enzyme activities showed an initial increase followed by a decreasing trend. The enzyme activities (SOD, CAT, LZM, ACP, AKP) generally were higher at 12 h and 24 h compared to the control group, and then decreased some at 48 h and 96 h. The richness and diversity of intestinal microbiota decreased under ammonia stress. Proteobacteria, Firmicutes, and Bacteroidetes dominated in both the control and the 96-h group. Principal component analysis indicated a relatively small alteration of the microbiota by ammonia. Chitinophagaceae and Chloroflexi were the main species in the 96-h group, while Clostridium sensu stricto 1 and Paracoccus were the main species in the control group. Our findings suggest that ammonia exposure suppressed the antioxidant and immunological defenses of Litopenaeus vannamei. Ammonia exposure induced variation in the expression of intestinal microbiota abundance and composition, but the changes were unremarkable.