Metatranscriptomics supports mechanism for biocathode electroautotrophy by "Candidatus Tenderia electrophaga"

Biocathodes provide a stable electron source to drive reduction reactions in electrotrophic microbial electrochemical systems. Electroautotrophic biocathode communities may be more robust than monocultures in environmentally relevant settings, but some members are not easily cultivated outside of the electrode environment. We previously used metagenomics and metaproteomics to propose a pathway for coupling extracellular electron transfer (EET) to carbon fixation in \"Candidatus Tenderia electrophaga\", an uncultivated but dominant member of the Biocathode-MCL electroautotrophic community. Here we validate and refine this proposed pathway using differential metatranscriptomics of replicate MCL reactors poised at the growth potential 310 mV and the suboptimal 470 mV (vs. standard hydrogen electrode). At both potentials, transcripts from \"Ca. Tenderia electrophaga\" were more abundant than from any other organism and its relative activity was positively correlated with current. Several genes encoding key components of the proposed \"Ca. Tenderia electrophaga\" EET pathway were more highly expressed at 470 mV, consistent with a need for cells to acquire more electrons to obtain the same amount of energy as at 310 mV. These included cyc2, encoding a homolog of a protein known to be involved in iron oxidation, confirmed to be differentially expressed by droplet digital PCR of independent biological replicates. Average expression of all CO2 fixation related genes is 1.23-fold higher at 310 mV, indicating that reduced energy availability at 470 mV decreased CO2 fixation. Our results substantiate the claim that \"Ca. Tenderia electrophaga\" is the key MCL electroautotroph, which will help guide further development of this community for microbial electrosynthesis.\n\nIMPORTANCEBacteria that directly use electrodes as metabolic electron donors (biocathodes) have been proposed for applications ranging from microbial electrosynthesis to advanced bioelectronics for cellular communication with machines. However, just as we understand very little about oxidation of analogous natural insoluble electron donors, such as iron oxide, the organisms and extracellular electron transfer (EET) pathways underlying the electrode-cell direct electron transfer processes are almost completely unknown. Biocathodes are a stable biofilm cultivation platform to interrogate both the rate and mechanism of EET using electrochemistry and study the electroautotrophic organisms that catalyze these reactions. Here we provide new evidence supporting the hypothesis that the uncultured bacterium \"Candidatus Tenderia electrophaga\" directly couples extracellular electron transfer to CO2 fixation. Our results provide insight into developing biocathode technology, such as microbial electrosynthesis, as well as advancing our understanding of chemolithoautotrophy.