Carbonate alkalinity disrupts the bile acid liver-gut axis and impairs growth in Carassius auratus

Carbonate alkalinity represents a primary constraint restricting the growth performance and nutrient utilization of aquaculture species in saline-alkaline waters. However, the specific physiological interplay by which this external chemical stressor disrupts metabolic homeostasis and uncouples the liver-gut axis remains poorly understood. This study combined physiological and histopathological assessments with integrated multi-omics analysis to elucidate the molecular drivers of growth retardation in Carassius auratus, a staple aquaculture species, under carbonate alkalinity stress. Chronic exposure to carbonate alkalinity resulted in significant growth inhibition in both the 1680 and 3360 mg/L NaHCO₃ groups, while the 3360 mg/L NaHCO₃ group exhibited more severe digestive impairment, as reflected by reduced lipase and amylase activities. Histopathological analysis revealed overt tissue damage, characterized by hepatic vacuolization with visible bile thrombi and gut mucosal atrophy. Physiologically, this was accompanied by a systemic collapse of serum lipid profiles (decreased triglycerides and total cholesterol) and a paradoxical increase in LDL-C. Integrated multi-omics analyses showed that carbonate alkalinity exposure was accompanied by a marked reduction in hepatic AE2 protein abundance, together with bile acid (BA) redistribution along the liver-gut axis. These changes were consistent with impaired bicarbonate-associated bile secretion and altered enterohepatic BA homeostasis. This secretory failure led to a depletion of luminal BAs, which consequently failed to trigger the negative feedback loop in the gut. This altered feedback pattern was associated with the paradoxical upregulation of hepatic BA biosynthesis-related enzymes (CYP8B1 and CYP27A1), driving a 15.7-fold (relative to the control group) intrahepatic accumulation of glycocholic acid. In the gut, reduced luminal BA availability was associated with lower abundance of lipid transport-related proteins (FATP4 and APOA1/4) and increased expression of the inflammatory mediator COX-2. Furthermore, the altered BA profile was associated with gut microbiota dysbiosis, characterized by Proteobacteria enrichment and Paenibacillus depletion. These findings suggest that AE2-associated BA transport dysfunction may represent an important mechanistic link connecting carbonate alkalinity stress with hepaticBA retention, gut BA depletion, and impaired lipid utilization. Collectively, these findings support a proposed model in which impaired enterohepatic BA homeostasis contributes to hepatic BA retention and gut lipid absorption dysfunction. Consequently, future nutritional intervention studies using exogenous BAs are warranted to test whether restoring BA homeostasis can improve metabolic resilience and production performance in saline-alkaline aquaculture.