Comparative metagenome analyses of anode-associated microbial communities developed in rice paddy field-soil microbial fuel cells

Sediment-type microbial fuel cells (sMFCs) can be operated in rice paddy fields, in which rice-root exudates are converted to electricity by anode-associated rhizosphere microbes. Previous studies have suggested members of the family Geobacteraceae being enriched on anodes of rhizosphere sMFCs. In order to deepen our understanding of rhizosphere microbes involved in electricity generation in sMFCs, the present study conducted comparative analyses of anode-associated microbes in three MFCs; namely, sMFC installed in a rice paddy field, and acetate- and glucose-fed MFC reactors in which pieces of anode graphite felts that had cut from a rice paddy-field sMFC were used as microbe-bearing anodes. After electric outputs became stable, microbial communities established in association with anodes of these MFCs and that in bulk soil were comparatively analyzed by pyrotag sequencing of 16S rRNA gene amplicons and Illumina shotgun metagenomics. The pyrotag sequencing showed the presence of Geobacteraceae bacteria in association with the anodes, while types of Geobacter that grew in these MFCs were different. Namely, those closely related to G. metallireducens shared 90% of the anode Geobacteraceae in the acetate-fed MFC, while, in the rhizosphere sMFC and glucose-fed MFC, such Geobacteraceae sequences were minor, and those closely related to G. psychrophilus were abundantly detected. This trend was also confirmed by phylogenetic assignments of genes predicted in shotgun metagenome sequences of the anode communities. We suggest that G. psychrophilus relatives preferentially grow on anodes of rhizosphere sMFCs and generate electricity under syntrophic interactions with organisms excreting electron donors.