Extracellular electron transfer genes expressed by candidate flockingbacteria in cable bacteria sediment

Cable bacteria, filamentoussulfideoxidizers that live in sulfidicsediments, are at times associated with large flocksof swimming bacteria. It has been proposed that these flocksof bacteria transport electrons extracellularly to cable bacteria via an electron shuttle intermediate, but the identity and activity of these bacteria in freshwater sediment remain mostly uninvestigated. Here, we elucidate the electron exchange capabilities of the bacterial community by coupling metagenomics and metatranscriptomics to 16S rRNA amplicon-based correlations with cable bacteria over 155 days. We identifiedcandidate flockingbacteria as bacteria containing genes for motility and extracellular electron transfer including synthesis genes for potential extracellular electron shuttles: phenazines and flavins.Based on these criteria, 22 MAGs were from candidate flockers,which constituted 21.4% of all 103 MAGs. Of the candidate flockingbacteria, 42.1% expressed extracellular electron transfer genes. The proposed flockersbelonged to a large variety of metabolically versatile taxonomic groups: 18 genera spread across nine phyla. Our data suggest that cable bacteria in freshwater sediments engage in electric relationships with diverse exoelectrogenic microbes. This community, found in deeper anoxic sediment layers, is involved in sulfur, carbon, and metal (in particular Fe) cycling and indirectly utilizes oxygen here by extracellularly transferring electrons to cable bacteria.