Different autotrophic enrichments yield efficient inocula for biocathodic applications

The uptake of extracellular electrons from cathodes by microbes is a recently discovered phenomenon. However, current knowledge on the diversity of microbes accepting extracellular electrons and populating biocathodes is scarce. Research is required to explore the distribution of extracellular electron uptake metabolism in the tree of life across microbial guilds. Here we characterize the electron uptake ability of microbial guilds enriched on H2, S2O32-, or CH4 and NH4+ from the same inoculum taken from a groundwater treatment sand-filter. We hypothesized that functional microbes having dense outer membrane cytochromes in their native pathway, such as pathways of NH4+, CH4, S2O32- and H2, can perform extracellular electron uptake. We aimed of addressing the following questions: (1) Are there any known microbial member of anticipated function performing extracellular electron transfer? (2) How does electron uptake efficiency vary between the different functional guilds? (3) How is the dissipated electron energy distributed across metabolisms? We developed and applied a novel pipeline to identify taxa utilizing direct electron energy and utilizing secondary microbial products. We report the putative direct electron uptake metabolism of types belonging to Methylomonas, UBA6140, and Nitrosomonas. Furthermore, members of Streptococcaceae, Rhizobiaceae, Streptococcus, Brevundimonas, Chryseobacterium, and Pseudomonas are detected as electroactive taxa. Our results reveal novel insights into the diversity, electrochemical activity, and metabolism of taxa performing direct electron uptake.