ABSTRACT Nitrite accumulation in aquaculture environments poses a critical threat to fish health, particularly affecting sensitive physiological systems such as the gut. This study investigated the intestinal stress response of juvenile four‐finger threadfin ( Eleutheronema tetradactylum ) under nitrite exposure through integrated analyses of intestinal microbiota and metabolomics. Acute toxicity tests revealed that nitrite has a time‐ and dose‐dependent effect, with a 96‐h LC 50 of 4.95 mg/L and a safe concentration estimated at 0.50 mg/L. Under nitrite stress, significant alterations were observed in the intestinal microbial community structure, notably a shift in the abundance of dominant genera such as Vibrio , Photobacterium and Bacillus . Although alpha diversity indices did not show significant variation, beta diversity analyses highlighted distinct microbial compositions among control, stress and recovery groups. Functional prediction using FAPROTAX suggested that nitrite stress altered microbial activities related to chemoheterotrophy and nitrate reduction. Metabolomic profiling identified substantial shifts in metabolite levels, with key enrichments in ABC transporter and aminoacyl‐tRNA biosynthesis pathways during stress and recovery, respectively. Enzyme assays further revealed that nitrite stress caused dynamic changes in intestinal amylase, lipase and trypsin activities, with partial recovery post‐exposure. The integration of microbial and metabolic data demonstrated strong correlations between specific microbial taxa and metabolites, suggesting coordinated host–microbe responses under nitrite‐induced oxidative and metabolic stress. These findings provide comprehensive insights into the gut‐level impact of nitrite toxicity and identify key microbial and metabolic biomarkers involved in stress adaptation and recovery. The study underscores the importance of nitrite monitoring and management to ensure intestinal health and sustainable aquaculture of E. tetradactylum .
Liu et al. (Fri,) studied this question.