Ammonia Nitrogen Stress Disrupts Respiratory and Immune System, Antioxidant Capacity, and Intestinal Microbiota in Juvenile Cichlid Pseudotropheus zebra

Abstract

As ammonia nitrogen pollution poses a critical threat to aquaculture sustainability and juvenile cichlid health remains poorly understood, this study investigated the physiological and microbiological responses of juvenile cichlid Pseudotropheus zebra (7.9 ± 0.5 g) to acute ammonia stress. The total ammonia nitrogen concentrations of 0 (control group), 5, and 12.25 mg/L were setup. Then, samples were collected at 0, 12, 24, 48, and 96 h ammonia nitrogen stress. The results showed that the indexes such as respiratory enzyme activity, immune, and antioxidant related enzyme in ammonia nitrogen stress were significantly (p < 0.05) changed than that of control group. After 96 h of ammonia nitrogen stress, morphology and structure index of gills, liver, and intestines in ammonia nitrogen groups were remarkably (p < 0.05) changed with compared to control group. Exposure to higher concentrations of ammonia nitrogen results in more severe damage, including intestinal swelling and increased of muscle layer thickness. There were significant (p < 0.05) differences in intestinal microorganisms between ammonia nitrogen stress and control group. The results of α-diversity analysis revealed that ammonia nitrogen stress significantly altered the abundance and community structure of intestinal microorganisms. The abundance of beneficial bacteria, such as Firmicutes, was reduced in ammonia nitrogen stress than that of control group. The activity of respiratory enzyme, immune, and antioxidant related enzyme were remarkably (p < 0.05) correlated with abundance changes of beneficial bacterial communities, including Acinetobacter, Blautia, and Lactobacillus. Overall, these findings provide novel insights into the interactions among nitrogen stress and intestinal microbiota and can guide practices for culture of juvenile cichlid Pseudotropheus zebra.