Hydrocarbon subsidy-stress and divergent reduced iron-manganese thresholds shape microbial resilience in anoxic aquifers.

The co-occurrence of geogenic metal mobilization and anthropogenic hydrocarbon contamination represents a pervasive hydro-biogeochemical challenge, yet the non-linear mechanisms governing microbial resilience in these multi-stressor aquifers remain poorly constrained. Here, we decode these complex interactions by integrating high-resolution microbiome profiling with an interpretable machine learning framework (XGBoost-SHAP). We identify a "multi-threshold superposition" model for total petroleum hydrocarbons (TPHs) that delineates a tri-phasic ecological transition: a progression from carbon limitation to metabolic subsidization, culminating in a toxicity-driven regime shift beyond a mechanistically grounded tipping point (3.485 mg/L). Crucially, we unveil a "Fe(II)-Mn(II) Paradox" wherein geochemically similar metals exert divergent ecological controls. Deviating from the additive toxicity paradigm, elevated Fe(II) (>9.22 mg/L) serves as a redox-mediated buffer against hydrocarbon stress; in contrast, Mn(II) operates as a toxicological synergist, amplifying metabolic and structural impairments beyond predicted additive levels. Mechanistically, this resilience is underpinned by the rare taxa acting as a functional seed bank, which reconfigures network topology from competitive exclusion to cooperative syntrophy. These findings establish the stoichiometric Fe(II)/Mn(II) ratio (threshold: 1.935) as a master regulator of aquifer, providing a quantitative foundation for "precision zoned intervention" strategies-advocating for monitored natural attenuation in iron-buffered zones versus active engineering in manganese-aggravated hotspots.