Direct interspecies electron transfer accelerates syntrophic oxidation of butyrate in paddy soil enrichments

Syntrophic interaction occurs during anaerobic fermentation of organic substances forming methane as the final product. H₂ and formate are known to serve as the electron carriers in this process. Recently, it has been shown that direct interspecies electron transfer (DIET) occurs for syntrophic CH₄ production from ethanol and acetate. Here, we constructed paddy soil enrichments to determine the involvement of DIET in syntrophic butyrate oxidation and CH₄ production. The results showed that CH₄ production was significantly accelerated in the presence of nanoFe₃O₄ in all continuous transfers. This acceleration increased with the increase of nanoFe₃O₄ concentration but was dismissed when Fe₃O₄ was coated with silica that insulated the mineral from electrical conduction. NanoFe₃O₄ particles were found closely attached to the cell surfaces of different morphology, thus bridging cell connections. Molecular approaches, including DNA‐based stable isotope probing, revealed that the bacterial Syntrophomonadaceae and Geobacteraceae, and the archaeal Methanosarcinaceae, Methanocellales and Methanobacteriales, were involved in the syntrophic butyrate oxidation and CH₄ production. Among them, the growth of Geobacteraceae strictly relied on the presence of nanoFe₃O₄ and its electrical conductivity in particular. Other organisms, except Methanobacteriales, were present in enrichments regardless of nanoFe₃O₄ amendment. Collectively, our study demonstrated that the nanoFe₃O₄‐facilitated DIET occurred in syntrophic CH₄ production from butyrate, and Geobacter species played the key role in this process in the paddy soil enrichments.