Effects of ammonia nitrogen stress on liver tissue structure and physiological indicators, and metabolomic analysis of juvenile four-finger threadfin (Eleutheronema tetradactylum)

Abstract

In intensive aquaculture, ammonia nitrogen (NH₃-N) is a major pollutant, causing oxidative stress and immune damage to aquatic organisms. The liver is crucial in protecting against biotic and abiotic stresses, but the response mechanisms to ammonia stress in juvenile four-finger threadfin (Eleutheronema tetradactylum) are not well understood. This study investigated these mechanisms by examining liver tissue structure, enzyme activities, and metabolomic changes in response to ammonia stress. Juvenile four-finger threadfin (7.4 ± 0.6 g) were divided into control, NH₃-N stress (50% LC50 96 h, 10 ± 0.4 mg/L), and postexposure recovery groups. Stress durations of 12, 24, 48, and 96 h were evaluated, followed by 48 h recovery. Prolonged ammonia stress led to increased liver tissue damage, including disordered hepatocyte arrangement, swelling, necrosis, and the disappearance of nucleoli. After 48 h recovery, liver damage was alleviated but did not fully return to control levels, suggesting that the toxic effects of ammonia are recoverable yet persistent. Antioxidant enzyme activities (superoxide dismutase, catalase, and glutathione peroxidase) initially showed significant increases peaking at 24 h after stress, before declining by 96 h. Malondialdehyde levels rose initially and remained elevated compared with controls. After 48 h of recovery, antioxidant enzyme activity had not returned to control levels, indicating inadequate recovery from ROS-induced stress. Metabolomic analysis revealed 1219 significantly different metabolites in the 96 h stress group, with increases in L-histidine, L-threonine, and cholesterol. In the recovery group, 904 metabolites differed from controls, with notable reductions in urea and choline. The key affected pathways included amino acid, lipid, and nucleotide metabolism. This study elucidates the toxic effects of ammonia nitrogen on juvenile four-finger threadfin and their adaptive responses through physiological and metabolomic changes, providing insights for aquaculture practices and breeding ammonia-tolerant strains.