Abstract:With the rapid development of high-density intensive aquaculture, increased stocking densities and accelerated accumulation of residual feed and feces have led to excessive ammonia nitrogen accumulation in water bodies. This phenomenon can induce hepatic damage in aquatic species, impair immune enzyme activity, disrupt intestinal morphology and microbiota, and reduce digestive enzyme vitality. As an emerging feed additive, postbiotics are gaining increasing attention in aquaculture. Defined as inactivated microorganisms and/or their components with a well-characterized genetic background (including or excluding metabolites) that confer health benefits to the host, postbiotics have demonstrated multifunctional properties such as immune modulation enhancement, metabolic promotion, intestinal epithelial barrier reinforcement, antioxidant activity, anticancer effects, anti-inflammatory actions, and gut microbiota balance. However, research on whether dietary postbiotic supplementation improves ammonia nitrogen stress resistance in pearl gentian grouper (Epinephelus fuscoguttatus ♀×Epinephelus lanceolatus ♂) remains limited. This study evaluated the effects of dietary postbiotics on growth, immunity, and intestinal health of pearl gentian grouper (initial body weight: 31.30 ± 0.64 g) under ammonia nitrogen stress.Postbiotics (liquid form, 30% concentration) were supplemented into basal diets at 0, 250, 750, and 2,000 mL/ton, forming a control group (N0) and three experimental groups (N250, N750, N2000). A total of 480 fish were randomly allocated into 12 flow-through aquaculture tanks (300 L), with 40 fish per tank. Each experimental diet was randomly assigned to three tanks. Fish were fed twice daily (09:00 and 17:00) at 2% of body weight for 8 weeks. After the feeding trial, 25 fish from each tank were transferred to a 24-hour aerated static water system for a 1-week ammonia nitrogen stress test (5 mg/L).Results showed that post-ammonia stress, survival rate, specific growth rate (SGR), and weight gain rate (WGR) in the N750 and N2000 groups were significantly higher than those in N0 (P < 0.05), while feed conversion ratio (FCR) was significantly lower (P < 0.05). Postbiotic-supplemented groups exhibited significantly elevated hepatic activities of superoxide dismutase (SOD), catalase (CAT), acid phosphatase (ACP), alkaline phosphatase (AKP), and lysozyme (LZM) compared to N0 (P < 0.05), alongside significantly reduced malondialdehyde (MDA) content (P < 0.05). Intestinal protease activity markedly increased in postbiotic groups, whereas lipase activity decreased significantly (P < 0.05). Compared to N0, postbiotic supplementation significantly enhanced intestinal villus length, with the N2000 group showing notably thicker intestinal muscularis (P < 0.05). All experimental groups displayed significantly increased intestinal villus absorption area relative to N0 (P < 0.05). Dietary postbiotics modulated gut microbiota composition by elevating the relative abundance of Firmicutes and Bacteroidetes while reducing Proteobacteria, thereby improving microbial functionality.Correlation analysis revealed that genera such as unidentified_Kapabacteriales and Methanolinea in the N750 group played positive regulatory roles in immune responses. However, in the high-dose group (N2000), although beneficial Bacillus abundance was maintained, a rebound of Vibrio (similar to N0 levels) and reduced phosphatase-related microbial communities (e.g., Erysipelotrichaceae_UCG-003) suggested potential disruption of microbial homeostasis due to excessive postbiotic supplementation.In conclusion, dietary postbiotics significantly enhanced growth performance, digestive enzyme activity, non-specific immunity, intestinal morphology, and microbiota composition in pearl gentian grouper, improving ammonia nitrogen stress resistance. This study provides theoretical insights into postbiotic-mediated mitigation of ammonia stress in grouper aquaculture, analyzing the impacts of ammonia exposure on fish growth, non-specific immunity, intestinal structure, and microbial ecology. By evaluating the role of postbiotics in enhancing stress tolerance, these findings offer critical data and foundational knowledge for advancing sustainable aquaculture practices and optimizing eco-friendly feed formulations for pearl gentian grouper.
