title: Metatranscriptomics Supports the Mechanism for Biocathode Electroautotrophy by "CandidatusTenderia electrophaga".
abstract: 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 the electrode environment. We previously used metagenomics and metaproteomics to propose a pathway for coupling extracellular electron transfer (EET) to carbon fixation in "CandidatusTenderia electrophaga," an uncultivated but dominant member of an electroautotrophic biocathode community. Here we validate and refine this proposed pathway using metatranscriptomics of replicate aerobic biocathodes poised at the growth potential level of 310 mV and the suboptimal 470 mV (versus the standard hydrogen electrode). At both potentials, transcripts were more abundant from "Ca.Tenderia electrophaga" than from any other constituent, 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 includedcyc2, encoding a homolog of a protein known to be involved in iron oxidation. Mean expression of all CO2fixation-related genes is 0.27 log2-fold higher at 310 mV, indicating that reduced energy availability at 470 mV decreased CO2fixation. Our results substantiate the claim that "Ca.Tenderia electrophaga" is the key electroautotroph, which will help guide further development of this community for microbial electrosynthesis.IMPORTANCEBacteria 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 to study the electroautotrophic organisms that catalyze these reactions. Here we provide new evidence supporting the hypothesis that the uncultured bacterium "CandidatusTenderia electrophaga" directly couples extracellular electron transfer to CO2fixation. Our results provide insight into developing biocathode technology, such as microbial electrosynthesis, as well as advancing our understanding of chemolithoautotrophy.
authors.affiliation: United States Naval Research Laboratory, Washington, DC, USA.
authors.forename: Brian J
authors.identifier: 0000-0002-3559-3892
authors.initials: BJ
authors.lastname: Eddie
authors.affiliation: United States Naval Research Laboratory, Washington, DC, USA.
authors.forename: Zheng
authors.initials: Z
authors.lastname: Wang
authors.affiliation: United States Naval Research Laboratory, Washington, DC, USA.
authors.forename: W Judson
authors.identifier: 0000-0003-3285-6754
authors.initials: WJ
authors.lastname: Hervey
authors.affiliation: United States Naval Research Laboratory, Washington, DC, USA.
authors.forename: Dagmar H
authors.identifier: 0000-0003-2325-7143
authors.initials: DH
authors.lastname: Leary
authors.affiliation: United States Naval Research Laboratory, Washington, DC, USA.
authors.forename: Anthony P
authors.identifier: 0000-0001-6192-888X
authors.initials: AP
authors.lastname: Malanoski
authors.affiliation: United States Naval Research Laboratory, Washington, DC, USA.
authors.forename: Leonard M
authors.initials: LM
authors.lastname: Tender
authors.affiliation: United States Naval Research Laboratory, Washington, DC, USA.
authors.forename: Baochuan
authors.identifier: 0000-0002-9484-0785
authors.initials: B
authors.lastname: Lin
authors.affiliation: United States Naval Research Laboratory, Washington, DC, USA.
authors.forename: Sarah M
authors.initials: SM
authors.lastname: Strycharz-Glaven
keywords: biocathode; electroautotroph; metatranscriptomics; microbial electrochemical system