Iron (Hydr)oxides Suppress Dissolved Organic Sulfur Synthesis in Acid Mine Drainage Environments by Inhibiting Organic Carbon Degradation and Assimilatory Sulfate Reduction

Abstract

Dissolved organic sulfur (DOS) in acid mine drainage (AMD) has attracted widespread attention due to its potential metal-complexation capacity. Given the little knowledge about DOS in AMD environments, this study employs high-resolution mass spectrometry and metagenomics to investigate its occurrence, synthesis pathways, and influencing factors. The results indicate that DOS exists at a mass spectrometry intensity-weighted relative abundance of 15.8–36.0% in AMD environments, mainly in CHOS and CHONS forms, with both levels higher than those found in soil, marine, and groundwater environments. DOS is dominated by highly unsaturated compounds, with highly oxygenated and unsaturated DOS being more stable and exhibiting greater mobility. Assimilatory sulfate reduction is the primary pathway for DOS synthesis, exhibiting gene abundance 4.5 times higher than dissimilatory sulfate reduction. Iron (hydr)oxides reduce the DOS content in AMD environments by adsorbing and immobilizing highly unsaturated compounds in sediments and inhibiting assimilatory sulfate reduction and organic carbon degradation involved in DOS synthesis. Overall, this study clarifies the molecular occurrence and fate of DOS and reveals a microbial–mineral synergistic regulation mechanism, providing a fundamental basis for understanding its molecular characteristics and potential metal-complexation relevance in AMD systems.