From acute lethality to chronic impairment: ammonia effects on growth, antioxidant defense, and intestinal microbiota in Carassius auratus

To investigate the tolerance thresholds of Carassius auratus under acute and chronic ammonia stress and evaluate the effects of ammonia exposure on growth performance, physiological functions, and intestinal microbial communities. First, the 96 h median lethal concentration (LC₅₀) of ammonia was determined for C. auratus (4.68 ± 0.12 g) using the standard acute toxicity bioassay. Subsequently, following the acute to chronic ratio (ACR) protocol for predicting acceptable no effect concentration levels, C. auratus (5.01 ± 0.53) g were divided into five treatment groups: control (< 0.001 mg/L TAN), 1.56 (LC₅₀/200), 3.12 (LC₅₀/100), 6.25 (LC₅₀/50), 12.50 (LC₅₀/25) mg/L TAN, and subjected to a 10-week chronic exposure experiment. The 96 h LC₅₀ of ammonia nitrogen for C. auratus was calculated as 312.44 mg/L TAN by linear interpolation (y = 0.149 7x + 3.227 5, R² = 0.985 2, where y is mortality percentage and x is TAN concentration in mg/L). Under chronic ammonia stress, concentrations exceeding 1.56 mg/L TAN adversely affected growth performance (decreased final body weight, weight gain rate, and specific growth rate, and increased feed conversion ratio), blood health (decreased serum total protein and triglycerides, and increased blood ammonia, glucose, high density lipoprotein, aspartate aminotransferase, and alanine aminotransferase), intestinal structure (villous breakage, cell vacuolation, and muscularis thickening), intestinal digestive enzyme activities (decreased protease activity and increased lipase activity), and intestinal microbial communities (declines in α diversity and abundance). Ammonia concentrations above 3.12 mg/L TAN caused mortality in C. auratus. Concentrations exceeding 6.25 mg/L TAN induced intestinal oxidative stress (reduced total antioxidant capacity, superoxide dismutase, and catalase activities, and elevated malondialdehyde content). Under chronic ammonia stress, Cetobacterium became the dominant genus in the intestinal microbiota of C. auratus. Environmental ammonia nitrogen levels exceeding 1.56 mg/L TAN inhibited the growth performance of C. auratus, while above 3.12 mg/L TAN led to death. In aquaculture practice, it is recommended that C. auratus not be exposed to total ammonia concentrations higher than 1.56 mg/L for prolonged periods. The findings provide fundamental data for fish stress physiology and a scientific basis for establishing safe ammonia nitrogen thresholds and improving water quality management in C. auratus aquaculture.