Long-term stress induced by low bicarbonate alkalinity causes intestinal oxidative stress, gut microbiota imbalance and metabolic disorder in Scylla paramamosain

To investigate the effects of low bicarbonate alkalinity on intestinal health of Scylla paramamosain, we systematically evaluated the growth survival, intestinal antioxidant enzyme activities, and expression profiles of genes involved in immunity, inflammation, apoptosis and osmoregulation under low alkalinity conditions. Meanwhile, integrated intestinal microbiome and metabolome analyses were performed. The results showed that low bicarbonate alkalinity significantly reduced the survival and molting rates of S. paramamosain. It markedly elevated the activities of intestinal SOD, CAT and GSH-Px, as well as the expression levels of inflammation-related genes (IL-16, RELISH, LITAF), apoptosis-related genes (CASPASE7, BAX, BCL2) and osmoregulation-related genes (NKA, NHE, Cl-/HCO₃-), while suppressing the expression of immune genes (proPO, ALF1, LZM) (P < 0.05). Low bicarbonate alkalinity disrupted the intestinal microbiota structure of S. paramamosain, accompanied by a decreased abundance of the phylum Bacteroidota and a significant increase in the abundance of the genus Klebsiella (P < 0.05). A total of 714 differential metabolites were identified by metabolomic analysis, which were mainly annotated into Fatty Acyls, Glycerophospholipids, and Steroids and steroid derivatives. Multiple carbohydrate metabolism pathways were significantly enriched and participated in the regulation of Fructose 6-Phosphate (F6P). Integrated microbiome-metabolome analysis revealed that N,N-dimethyl arachidonoyl amine (NDAA) may cooperate with intestinal microorganisms to regulate the stress tolerance of S. paramamosain under low bicarbonate alkalinity stress. Based on the physiological and biochemical characterization of the intestine, this study elucidated the adaptive characteristics of S. paramamosain under low bicarbonate alkalinity. The findings provide important theoretical basis and practical significance for the saline-alkaline aquaculture of S. paramamosain.