Toxic effects of ammonia and thermal stress on the intestinal microbiota and transcriptomic and metabolomic responses of Litopenaeus vannamei

Ammonia and thermal stress frequently have harmful effects on aquatic animals. The intestine is an important barrier allowing the body to defend against stress. In this study, we investigated the intestinal microbiota and transcriptomic and metabolomic responses of Litopenaeus vannamei subjected to individual and combined ammonia and thermal stress. The results showed that obvious variation in the intestinal microbiota was observed after stress exposure, with increased levels of Firmicutes and decreased levels of Bacteroidetes and Planctomycetes. Several genera of putatively beneficial bacteria (Demequina, Weissella and Bacteroides) were abundant, while Formosa, Kriegella, Ruegeria, Rhodopirellula and Lutimonas were decreased; pathogenic bacteria of the genus Vibrio were increased under individual stress but decreased under combined stress. The intestinal transcriptome revealed several immune-related differentially expressed genes associated with the peritrophic membrane and antimicrobial processes in contrasting accessions. Haemolymph metabolomic analysis showed that stress exposure disturbed the metabolic processes of the shrimp, especially amino acid metabolism. This study provides insight into the underlying mechanisms associated with the intestinal microbiota, immunity and metabolism of L.vannamei in response to ammonia and thermal stress; ten stress-related metabolite markers were identified, including L-lactic acid, gulonic acid, docosahexaenoic acid, l-lysine, gamma-aminobutyric acid, methylmalonic acid, trans-cinnamate, N-acetylserotonin, adenine, and dihydrouracil.