Natural organic matter drives iodine biogeochemical cycling in multi-layered redox dynamic groundwater systems

The iodine (I) enrichment of groundwater is observed under wide redox conditions that are controlled by complex hydro-biogeochemical processes. However, the dominant processes across different redox gradients remain unclear. In this study, a field site having a high I concentration was selected, and multi-level monitoring wells [20 m × 30 m × 40 m × 50 m (depth)] were established. The groundwater chemistry was determined, and organic matter (OM) fluorescence and molecular analyses, as well as metagenomics analyses, were performed to identify the dominant biogeochemical processes controlling I enrichment in groundwater. In suboxic environments, phenolic and saturated hydrocarbon-rich aromatic OM provided energy for nitrogen-fixing and dehalogenating microbes, facilitating I desorption from Fe-NOM complexes. Degradation products, such as small organic acids, supported nitrate reduction (narL, narBHY, and nirS) and iodide formation. Under anoxic conditions, Desulfovibrio and dissimilatory sulfate reduction (aprAB and dsrAB) enhanced I enrichment through desorption, deiodination, and reduction, supported by pathways involving saturated and aliphatic compounds. Under strictly anoxic conditions, fermentative bacteria decomposed complex OM, promoting Fe-NOM dissolution and supplying substrates for iron-reducing bacteria, leading to extensive I release. Adsorption by secondary iron sulfides and methylation by methyltransferases (mtrABC and mtaA) partially restricted I concentrations. This study presents a theoretical model that clarifies microbial I mobilization mechanisms in groundwater, highlighting OM degradation prioritization as a key constraint.