Effects of Seawater from Different Sea Areas on Abalone Gastrointestinal Microorganisms and Metabolites

Significant regional variations in seawater characteristics (temperature, salinity, pH, nutrients) exist across marine environments, yet their impacts on abalone gastrointestinal microbiota and metabolites remain underexplored. This study investigated seawater nutrient and pH interactions on abalone gut ecosystems through comparative analysis of three marine regions (Pingtan (PT), Xiapu (XP), Lianjiang (LJ)). Seawater characteristics revealed distinct patterns: LJ exhibited the lowest total phosphorus (TP: 0.12 mg/L), total nitrogen (TN: 2.8 mg/L), NH₃-N (0.05 mg/L) but the highest salinity (32.1‰) and lowest pH (7.82), while PT/XP showed elevated nutrients (TP: 0.24–0.28 mg/L; TN: 4.2–4.5 mg/L). Microbial diversity peaked in LJ samples (Shannon index: 5.8) with dominant genera <i>Psychrilyobacter</i> (12.4%) and <i>Bradyrhizobium</i> (9.1%), contrasting with PT’s <i>Mycoplasma</i>-enriched communities (18.7%) and XP’s <i>Vibrio</i>-dominant profiles (14.3%). Metabolomic analysis identified 127 differential metabolites (VIP > 1.5, <i>p</i> < 0.05), predominantly lipids (38%) and organic acids (27%), with pathway enrichment in sulfur relay (q = 4.2 × 10⁻⁵) and tryptophan metabolism (q = 1.8 × 10⁻⁴). Stomach-specific metabolites correlated with fatty acid degradation (e.g., inosine diphosphate, r = −0.82 with vibrionimonas) and glutathione metabolism (methionine vs. mycoplasma, r = −0.79). Critically, pH showed negative correlations with beneficial <i>Psychrilyobacter</i> (oleamide: r = −0.68) and positive associations with pathogenic <i>Vibrio</i> (trigonelline: r = 0.72). Elevated NH₃-N (>0.15 mg/L) and TP (>0.25 mg/L) promoted <i>Mycoplasma</i> proliferation (R² = 0.89) alongside cytotoxic metabolite accumulation. These findings demonstrate that higher pH (>8.0) and nutrient overload disrupt microbial symbiosis, favoring pathogens over beneficial taxa.