Dissolved oxygen levels differentially regulate ammonia metabolism plasticity in mandarin fish (Siniperca chuatsi)

Dissolved oxygen (DO) is a key environmental factor affecting fish metabolism, yet its regulatory role in ammonia production, conversion, and excretion remains poorly understood. This study investigated how DO levels modulate ammonia metabolism plasticity in mandarin fish (Siniperca chuatsi) as a potential biological strategy for mitigating aquaculture-derived ammonia pollution. Fish were exposed to normoxia (6.05 ± 0.17 mg/L), hypoxia (2.67 ± 0.25 mg/L), or hyperoxia (9.73 ± 0.62 mg/L) for 48 h. Results showed that hypoxia caused oxidative stress, as evidenced by elevated MDA levels, suppressed SOD and CAT activities, and obvious tissue damage to the livers and gills of S. chuatsi. In response to this stress, both hypoxia and hyperoxia enhanced the antioxidant capacity of S. chuatsi via the Nrf2/Keap1 pathway. Compared to the normoxia and hyperoxia groups, the ammonia excretion rate was significantly lower in the hypoxia group. Distinct adaptive strategies in ammonia homeostasis were observed under different DO conditions. Under hypoxic condition, S. chuatsi downregulated hepatic glutamate dehydrogenase (gdh) to reduce ammonia production, upregulated carbamyl phosphate synthase 1 (cps1) to promote ammonia conversion to urea, and upregulated RH glycoproteins and Na⁺/H⁺ exchanger 3 (nhe3) as a compensatory response, ultimately leading to a 24.06% reduction in water NH₄ ⁺-N accumulation at 48 h. In contrast, hyperoxia reduced ammonia production and increased urea conversion but did not enhance excretion. Instead, it improved ammonia tolerance in S. chuatsi. In summary, hypoxia reduced net ammonia excretion via integrated adjustments in production, conversion, and excretion, whereas hyperoxia enhanced ammonia tolerance without altering excretion. This DO-mediated metabolic plasticity provides a biological strategy for mitigating aquaculture-derived ammonia pollution by utilizing existing aeration to harness intrinsic fish physiology, supporting the development of sustainable aquaculture.