Ammonia nitrogen accumulation and metabolic regulation of Paramisgurnus dabryanus exposed to sub-chronic carbonate alkalinity: Insights from biochemical and omics analyses

High carbonate alkalinity (CA) is a defining characteristic of saline-alkali waters, driving the ammonia nitrogen (NH3 and NH4+) equilibrium reaction toward NH3. Freshwater fishes are susceptible to ammonia poisoning in saline-alkali conditions since the impeded diffusion of endogenous NH3. In this study, large-scale loach (Paramisgurnus dabryanus) were exposed to 15 and 30 mmol/L NaHCO3 for 28 d, combined with biochemical, transcriptomic, and metabolomic approaches for better comprehension of the ammonia nitrogen accumulation and metabolic regulation mechanisms in freshwater fishes during saline-alkaline adaptation. The results showed that CA exposure significantly lowered ammonia excretion rate while briefly increased oxygen consumption rate and O/N ratio in fish, followed by partial recovery. Serum and tissue ammonia levels, glutamine synthetase (GS), and transaminase (AST and ALT) activities were significantly elevated, while trypsin (TPS) activity was significantly reduced, with 30 mmol/L NaHCO3 exerting a greater impact. There were 2740 differentially expressed genes and 323 differentially expressed metabolites identified in the livers of the 30 mmol/L NaHCO3 group compared to the control group, involved in protein degradation, amino acid metabolism, aminoacyl-tRNA biosynthesis, ribosome biogenesis, and carbohydrate metabolism. Notably, most nitrogenous compound catabolism-related genes, short peptides, and free amino acid levels were significantly down-regulated. Furthermore, Amide biosynthetic process and Alanine, aspartate, and glutamate metabolism could be key pathways associated with endogenous ammonia detoxification. The integrated omics analysis demonstrated activation of TCA cycle and Glycolysis/Gluconeogenesis pathways. These results suggest that large-scale loach might enhance glutamine synthesis and transamination to fix ammonia accumulated in vivo, as well as regulating carbohydrate metabolism to maintain energy homeostasis when protein and amino acid metabolism is disturbed. This study offers vital clues into large-scale loach adaptation mechanisms to saline-alkali environments and a novel perspective for molecular breeding of saline-alkaline-tolerant varieties.