The use of thermophilic microorganisms as biocatalysts for electromethanogenesis was investigated. Single-chamber reactors inoculated with thermophiles and operated at 55 °C showed high CH4 production rates (max. 1103 mmol m(–2) day(–1) at an applied voltage of 0.8 V) with current-capture efficiencies >90%, indicating that thermophiles have high potential as biocatalysts. To improve the electromethanogenic activity, the developed biocathode was transferred to a two-chamber reactor and operated at a poised potential of −0.5 V vs SHE. The CH4 production rates of the biocathode were enhanced approximately 6-fold in 160 h of poised-potential incubation, indicating that the acclimation of the biocathode resulted in performance improvement. Compositional alteration of the cathodic microbiota suggested that a Methanothermobacter-related methanogen and synergistetes- and thermotogae-related bacteria were selected during the acclimation. Cyclic voltammetry of the “acclimated” biocathode showed an augmented cathodic catalytic wave with a midpoint potential at ca. −0.35 V vs SHE. Moreover, the biocathode was able to catalyze electromethanogenesis at −0.35 V vs SHE. These results suggested that the ability of the biocathode to catalyze electromethanogenesis via direct electron transfer was enhanced by the acclimation. This study provides new technological and fundamental information on electromethanogenic bioelectrochemical systems (BESs) that may be extended to other BESs.

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To exploit the potential diversity of thermophilic exoelectrogens, two-chamber microbial fuel cells (MFCs) were inoculated with thermophilic anaerobic digester sludge and operated at 55 °C without supplementing with exogenous redox mediator. The MFC generated a maximum power density of 823 mW m(-2) after 200 h of operation. Molecular phylogenetic analyses suggested that the microbial population on the anode was dominated by a species closely related to a thermophilic nitrate-reducing bacterium Calditerrivibrio nitroreducens, for which a strain (Yu37-1) has been isolated in pure culture. Thus, a pure culture of the C. nitroreducens strain Yu37-1 was inoculated into MFC to examine the electricity generation capability. Without an exogenous mediator, MFCs stably produced electricity with a maximum power density of 272 mW m(-2) for >400 h of operation. The MFC current recovered to the original level within few hours after medium replacement, suggesting that the electricity generation was caused by the anodic microorganisms. Cyclic voltammetry indicated that redox systems (E3 and Ec) with similar potentials (-0.14 and -0.17 V) made the main contributions to the exoelectrogenic activities of the sludge-derived consortium and C. nitroreducens Yu37-1, respectively. This study undertook the bioelectrochemical characterization of C. nitroreducens as the first example of a thermophilic Gram-negative exoelectrogen.