Iron and sulfate reduction vs. methanogenesis: Contrasting terminal anaerobic organic matter degradation pathways regulate groundwater iodine enrichment.

Groundwater iodine enrichment is fundamentally controlled by contrasting natural organic matter degradation pathways, yet their molecular-scale regulatory mechanisms remain unresolved. This study integrates stable carbon isotopes, excitation-emission matrix (EEM) fluorescence spectroscopy, and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to decipher pathway-specific drivers in the Yangtze River alluvial aquifer. Integrated geochemical evidence reveals two distinct regimes: iron and sulfate reduction-dominated systems exhibit strong δ¹³C-DIC depletion (down to -16.7‰) with elevated dsrB gene abundance (mean 3.79 × 10⁶ copies/L), while methanogenesis-dominated systems show pronounced δ¹³C-DIC enrichment (up to +10.5‰) with low dsrB levels (mean 7.08 × 10⁵ copies/L). Dissolved organic matter (DOM) in iron- and sulfate- reducing zones is characterized by 25.0% sulfur-enriched compounds and fulvic-like fluorescence, driving iodine release through microbially mediated Fe(III)/sulfate reduction. Biogenic HS⁻ from sulfate reduction subsequently binds unsaturated organics to form persistent CHOS compounds. Conversely, methanogenesis-dominated DOM accumulates 14.8% nitrogen-rich, highly unsaturated/phenolic compounds; here, thermodynamically favored degradation of oxidized humic substances releases iodine alongside methane and ammonium. This work underscores the significance of nitrogen-rich macromolecule degradation (methanogenesis) and sulfur-rich organic generation (driven by iron and sulfate reduction) as critical, previously overlooked regulators of iodine mobility, providing a novel mechanistic framework for elucidating the fundamental processes governing regional-scale iodine enrichment patterns in groundwater systems.