Conservation of Energetic Pathways for Electroautotrophy in the Uncultivated Candidate Order <i>Tenderiales</i>.

Electromicrobiology can be used to understand extracellular electron uptake in previously undescribed chemolithotrophs. Enrichment and characterization of the uncultivated electroautotroph "<i>Candidatus</i> Tenderia electrophaga" using electromicrobiology led to the designation of the order <i>Tenderiales.</i> Representative <i>Tenderiales</i> metagenome-assembled genomes (MAGs) have been identified in a number of environmental surveys, yet a comprehensive characterization of conserved genes for extracellular electron uptake has thus far not been conducted. Using comparative genomics, we identified conserved orthologous genes within the <i>Tenderiales</i> and nearest-neighbor orders important for extracellular electron uptake based on a previously proposed pathway from "<i>Ca.</i> Tenderia electrophaga." The <i>Tenderiales</i> contained a conserved cluster we designated <i>uetABCDEFGHIJ</i>, which encodes proteins containing features that would enable transport of extracellular electrons to cytoplasmic membrane-bound energy-transducing complexes such as two conserved cytochrome <i>cbb</i>₃ oxidases. For example, UetJ is predicted to be an extracellular undecaheme <i>c</i>-type cytochrome that forms a heme wire. We also identified clusters of genes predicted to facilitate assembly and maturation of electron transport proteins, as well as cellular attachment to surfaces. Autotrophy among the <i>Tenderiales</i> is supported by the presence of carbon fixation and stress response pathways that could allow cellular growth by extracellular electron uptake. Key differences between the <i>Tenderiales</i> and other known neutrophilic iron oxidizers were revealed, including very few Cyc2 genes in the <i>Tenderiales</i>. Our results reveal a possible conserved pathway for extracellular electron uptake and suggest that the <i>Tenderiales</i> have an ecological role in coupling metal or mineral redox chemistry and the carbon cycle in marine and brackish sediments. <b>IMPORTANCE</b> Chemolithotrophic bacteria capable of extracellular electron uptake to drive energy metabolism and CO₂ fixation are known as electroautotrophs. The recently described order <i>Tenderiales</i> contains the uncultivated electroautotroph "<i>Ca.</i> Tenderia electrophaga." The "<i>Ca.</i> Tenderia electrophaga" genome contains genes proposed to make up a previously undescribed extracellular electron uptake pathway. Here, we use comparative genomics to show that this pathway is well conserved among <i>Tenderiales</i> spp. recovered by metagenome-assembled genomes. This conservation extends to near neighbors of the <i>Tenderiales</i> but not to other well-studied chemolithotrophs, including iron and sulfur oxidizers, indicating that these genes may be useful markers of growth using insoluble extracellular electron donors. Our findings suggest that extracellular electron uptake and electroautotrophy may be pervasive among the <i>Tenderiales</i>, and the geographic locations from which metagenome-assembled genomes were recovered offer clues to their natural ecological niche.