Ammonia Stress Disrupts Intestinal Health in Litopenaeus vannamei Under Seawater and Low-Salinity Environments by Impairing Mucosal Integrity, Antioxidant Capability, Immunity, Energy Metabolism, and Microbial Community

Ammonia is a key water quality factor limiting shrimp aquaculture. Intestinal health is closely associated with the nutrition, metabolism and immunity of shrimp. However, the response characteristics of the shrimp intestine to ammonia stress under seawater and low-salinity environments remain unclear. In this study, the shrimp <i>Litopenaeus vannamei</i> reared in seawater (salinity 30) or low-salinity (salinity 3) water were subjected to ammonia stress for 14 days, respectively. The changes in intestinal morphology, antioxidant capacity, immune response, energy metabolism, and microbial community were systematically investigated. The results showed that ammonia stress induced intestinal tissue damage in both seawater and low-salinity cultured shrimp, characterized by epithelial cell detachment and mucosal structural disruption. At the molecular level, ammonia stress triggered intestinal stress responses by interfering with key physiological processes such as antioxidant defense and endoplasmic reticulum stress. This process further led to varying degrees of disorders in physiological functions, including immune regulation, inflammatory response, and autophagic activity. In addition, ammonia stress disrupted the homeostatic balance of intestinal energy metabolism by affecting the expression of genes related to glucose metabolism, the tricarboxylic acid (TCA) cycle, and mitochondrial respiratory chain. In addition, ammonia stress increased the diversity of intestinal microbiota and caused microbial dysbiosis by increasing harmful bacteria (e.g., <i>Vibrio</i>) and decreasing beneficial bacterial groups (e.g., <i>Bacillus</i>). Ammonia stress generally enhanced intestinal microbiota chemotaxis. Specifically, predicted functions of microbiota in seawater-cultured shrimp showed increased carbohydrate, linoleic acid, and cofactor/vitamin metabolism; in low-salinity-cultured shrimp, functions including protein digestion/absorption, flavonoid/steroid hormone biosynthesis, and glycosaminoglycan degradation were reduced. These results revealed that ammonia stress compromised shrimp intestinal health by disrupting mucosal structure, triggering stress responses, and disturbing immune function, energy metabolism, and microbial homeostasis. Notably, low-salinity cultured shrimp exhibited more pronounced intestinal stress responses and greater physiological vulnerability than seawater-cultured counterparts.