Microbial Reduction-Induced Selective DOM Transformation Governs As(III) Oxidation under Redox Oscillations.

Microbial respiration-triggered reductive transformation of dissolved organic matter (DOM) plays a central role in O2 activation and, consequently, As(III) oxidation under fluctuating hydrological conditions. However, the key DOM components and their molecular transformations associated with this electron-transfer process remain poorly resolved. Here, by integrating Fourier transform ion cyclotron resonance mass spectrometry with machine learning, we reveal that fulvic acid (FA), the predominant DOM fraction, undergoes selective molecular transformation by Geobacter sulfurreducens PCA, forming a reactive subpool that drives O2 activation and thereby enhances As(III) oxidation by approximately 2-10 fold. Reaction network analysis reveals that oxygen-rich aromatic precursors are preferentially transformed into more reduced intermediates through decarboxylation, dehydrogenation, and partial reduction pathways. These transformations collectively shift the molecular composition toward lower oxidation states and enhanced redox reactivity. Machine-learning analysis further identifies nitrogen- and sulfur-containing molecules with low oxidation states as the key components governing the overall redox activity of the system. These species represent a functionally distinct fraction of microbially transformed FA with enhanced electron-donating capacity. This study provides molecular-level insights into how microbial DOM transformation regulates O2 activation and As(III) oxidation, offering a mechanistic basis for predicting and manipulating redox reactivity in dynamic soil and sediment systems.