A Thermophilic Gram-Negative Nitrate-Reducing Bacterium, Calditerrivibrio nitroreducens, Exhibiting Electricity Generation Capability

Abstract

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.

Bioelectrochemical analysis of a hyperthermophilic microbial fuel cell generating electricity at temperatures above 80 °C

“…The maximum current during the voltammetry of the MFC anode was around 4 mA, which was lower than that of thermophilic MFCs (operated at 55°C) of identical design. 9) In addition, both the onset potential (−0.25 V versus Ag/AgCl) and midpoint potential (−0.15 V versus Ag/AgCl) during the voltammetry of the MFC were more positive than those of thermophilic MFCs, 9,11) meaning relatively weaker generation of the current by the MFCs in the present study. Nonetheless, the voltammogram of the uninoculated control showed only a background level current.…”

“…9) Each MFC reactor consisted of two glass bottles (300 mL) separated by a proton exchange membrane with a diameter of 4 cm (Nafion 117, DuPont Co., Wilmington, USA). The proton exchange membranes were serially boiled with 10% H 2 O 2 , deionized water, and 0.5 M H 2 SO 4 solution, as described previously.…”

“…8) Several studies have shown that thermophilic MFCs, which are inoculated with thermophilic microorganisms (with optimal growth temperature between 45 and 80°C), exhibit good performance at operating temperatures approximately 55-60°C. 6,[9][10][11][12][13] However, compared to mesophilic systems, MFCs with higher operation temperatures remain underexploited. For example, exoelectrogenic activity has been reported in more than 20 bacterial species, but most are mesophilic species affiliated with the phylum Proteobacteria.…”

“…For example, exoelectrogenic activity has been reported in more than 20 bacterial species, but most are mesophilic species affiliated with the phylum Proteobacteria. 3,14) On the other hand, only three thermophilic species, Thermincola potens strain JR, 15) Thermincola ferriacetica, 11) and Calditerrivibrio nitroreducens, 9) have been shown to generate electricity in an MFC.…”

Electrically Conductive Pili from Pilin Genes of Phylogenetically Diverse Microorganisms

“…For example, Calditerrivibrio nitroreducens is capable of extracellular electron transfer to electrodes (Fu et al 2013) and the density of aromatic amino acids in the C. nitroreducens pilin protein is higher than in the G. sulfurreducens pilin (Figure 1). However, the C. nitroreducens pilin contains nearly twice as many amino acids as the G. sulfurreducens pilin.…”