Ammonia stress induces hepatotoxicity in hybrid grouper: histological, oxidative, inflammatory, and metabolomic responses.

Ammonia acts as a key environmental pollutant in fish farming. Elevated ammonia concentrations can induce stress responses in fish, threatening their survival and growth. As a vital metabolic organ, the fish liver plays key roles in detoxification and immune defense. In the present study, juvenile hybrid groupers (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂) were subjected to ammonia stress at 4 mg/L (A4) and 8 mg/L (A8) over a 7-day period. Subsequently, multiple physiological aspects of the liver were assessed across several biological levels, encompassing histomorphology, oxidative stress, inflammatory factors, and metabolic profiles. The results showed that ammonia stress caused hepatic morphological alterations and triggered oxidative stress and inflammatory responses. In detail, MDA content and T-AOC and SOD activities were increased in the A4 and A8 groups (P < 0.05), and LPO content was increased in the A8 group (P < 0.05). Meanwhile, the mRNA expression levels of nrf2, trx, tnfα, and il1β genes were upregulated in the A4 and A8 groups (P < 0.05); gpx and il8 genes were upregulated but il10 gene was downregulated in the A8 group (P < 0.05); nqo1 gene was downregulated in the A4 group (P < 0.05). Furthermore, the hepatic metabolite profiles were perturbed, particularly "histidine metabolism" and "biosynthesis of valine, leucine, and isoleucine" pathways. Additionally, several functional metabolites, including L-tryptophan, indole, carnosine, citric acid, sphingosine, pantothenic acid, and inositol, were emerged as potential candidate biomarkers. These findings demonstrated that ammonia stress could induce physiological dysfunction in the liver of groupers through a cascade of tissue damage, activated oxidative stress, disrupted inflammatory responses and altered metabolic profiles, and such adverse effects were markedly aggravated with the increase in ammonia concentration. Future research should focus on long-term ammonia exposure, concentration-time dose effects, and multi-omics integration to clarify the underlying response mechanisms.