Microbial electrotrophs-driven molecular transformations of dissolved organic matter in paddy soil.

Microbial electrotrophs are key players in biogeochemical cycles, but the impact of electrotrophic communities on the diverse chemical composition and properties of dissolved organic matter (DOM) molecules in paddy soils has not been comprehensively explored. Herein, we investigated the response of DOM molecules in paddy soil-based microcosms to electrotrophic communities using microbial electrosynthesis systems (MESs), high-resolution mass spectrometry, and genome-centric metagenomics techniques. Compared to the open-circuit control, the closed-circuit system exhibited a 2.6-fold increase in dissolved organic carbon concentration after 120 days of incubation in the MESs, with aromatic and tannin molecular abundances rising by 3.5-fold and 4.4-fold, respectively. These results indicate that electrotrophic activity enhances both the structural complexity and aromatization (humification degree) of soil DOM. Functional annotation revealed significant enrichment of the reductive tricarboxylic acid (rTCA) cycle and Calvin-Benson-Bassham (CBB) cycle, demonstrating active CO assimilation by electrotrophs into complex organic compounds. Electrotrophic genera such as Pseudomonas, Hyphomicrobium, Phenylobacterium, Achromobacter, Geobacter, Anaeromyxobacter, and Magnetospirillum were substantially enriched under the closed-circuit conditions, with relative abundances increasing from 0.02-0.72 % to 1.65-13.68 %. These microbes likely facilitated DOM stabilization by coupling extracellular electron uptake with CO fixation, thereby transforming labile organic carbon into more stable molecular structures. These findings elucidate the impact of electrotrophic bacteria in regulating the DOM transformation, providing a deeper understanding of the transformation mechanisms of DOM in paddy soils.