Underlying the Enhanced Ammonia Stress Resistance by Prebiotic Xylo-Oligosaccharides in Hybrid Groupers: An Integrative Analysis of Serum Physiology, Hepatic Transcriptomics, and Intestinal Health

Elevated ammonia nitrogen restricts sustainable hybrid grouper aquaculture. Though certain prebiotics relieve ammonia stress in groupers, their regulatory mechanisms remain unclear. This work investigated the protective effects and molecular mechanisms of xylo-oligosaccharides (XOSs) against ammonia stress in hybrid groupers (Epinephelus fuscoguttatus ♀ × E. lanceolatus ♂) via a 4-week feeding trial (control diet/XOS-supplemented diet) followed by a 96 h ammonia challenge. Dietary XOS significantly boosted fish growth, increasing weight gain and specific growth rate while lowering feed conversion ratio (p < 0.05). During ammonia exposure, XOS-fed fish exhibited markedly higher survival from 48 to 96 h, with a 40% survival gap between the two groups at 96 h. XOS elevated serum glutathione peroxidase and lysozyme activities, suppressed catalase, aspartate transaminase and alanine transaminase levels (p < 0.05), strengthening antioxidant capacity, non-specific immunity and hepatic health. Transcriptomics identified differentially expressed genes enriched in nitrogen metabolism, steroid biosynthesis and complement cascades, verified by qPCR. XOS mitigated ammonia-triggered intestinal injury and reshaped gut microbiota: XOS supplementation enriched beneficial genera (Bifidobacterium, Turicibacter) and suppressed pathogenic taxa (Clostridium_sensu_stricto_1, Photobacterium) under ammonia stress. Collectively, dietary XOS improves hybrid groupers growth and ammonia resistance by regulating antioxidant enzymes, immune-related genes, intestinal morphology and gut microflora. This study offers theoretical support for developing anti-ammonia feed additives for grouper farming.