Molecular transformation of dissolved organic matter in paddy soils driven by electroactive microorganisms

Electroactive microorganisms play important roles in extracellular electron transfer and soil carbon cycling. However, their effects on the molecular transformation of dissolved organic matter (DOM) in paddy soils remain poorly understood. Here, a bioelectrochemical system (BES) was used to enrich electroactive microorganisms from paddy soil, and DOM molecular transformation together with microbial community dynamics was characterized by ultrahigh-resolution Orbitrap mass spectrometry, 16S rRNA gene amplicon sequencing, and co-occurrence network analysis. Compared with the open-circuit control, enrichment of electroactive microorganisms reduced the aromaticity of soil DOM and promoted the relative accumulation of aliphatic compounds. As current generation reached a stable stage, the relative abundances of unsaturated hydrocarbons and condensed aromatic compounds decreased, whereas those of protein-/aliphatic-like compounds, carbohydrates, and lignin/carboxyl-rich alicyclic molecules increased, indicating substantial molecular transformation of soil DOM during extracellular electron transfer. Electroactive microorganisms, together with hydrocarbon- and polycyclic aromatic hydrocarbon-degrading bacteria, were significantly enriched during BES cultivation. Co-occurrence network analysis further revealed that DOM transformation was not driven solely by electroactive microorganisms but was achieved through their synergistic interactions with other functional microorganisms, facilitating the transformation of structurally complex DOM into more bioavailable low-molecular-weight compounds. These findings provide new insights into the microbial mechanisms underlying DOM transformation during extracellular electron transfer and improve our understanding of soil organic carbon cycling in paddy ecosystems.