Production of electrically-conductive nanoscale filaments by sulfate-reducing bacteria in the microbial fuel cell.

This study reports that the obligate anaerobic microorganism, Desulfovibrio desulfuricans, a predominant sulfate-reducing bacterium (SRB) in soils and sediments, can produce nanoscale bacterial appendages for extracellular electron transfer. These nanofilaments were electrically-conductive (5.81S·m(-1)) and allowed SRBs to directly colonize the surface of insoluble or solid electron acceptors. Thus, the direct extracellular electron transfer to the insoluble electrode in the microbial fuel cell (MFC) was possible without inorganic electron-shuttling mediators. The production of nanofilaments was stimulated when only insoluble electron acceptors were available for cellular respiration. These results suggest that when availability of a soluble electron acceptor for SRBs (SO4(2-)) is limited, D. desulfuricans initiates the production of conductive nanofilaments as an alternative strategy to transfer electrons to insoluble electron acceptors. The findings of this study contribute to understanding of the role of SRBs in the biotransformation of various substances in soils and sediments and in the MFC.

Enhanced current production by Desulfovibrio desulfuricans biofilm in a mediator-less microbial fuel cell.

In this study, a mediator-less microbial fuel cell (MFC) inoculated with a sulfate-reducing bacterium (SBR), Desulfovibrio desulfuricans, was equipped with bare and surface-treated graphite felt electrodes. Electrochemical treatment of the anode surface facilitated biofilm formation on the electrode, resulting in rapid and enhanced current production. The maximum current density of the treated anode was 233±24.2mA/m(2), which was 41% higher than that of the untreated anode. The electron transfer rate also increased from 2.45±0.04 to 3.0±0.02µmol of electrons/mg of protein·min. Biofilm formation on the treated anode was mainly due to the strong hydrogen or peptide bonds between the amide groups of bacterial materials (including cytochrome c) and carboxyl groups formed on the electrodes. These results provide useful information on direct electron transfer by SRB in a mediator-less MFC through cytochrome c and the effects of the electrochemical treatment of electrodes on MFC performance.

Control of malodorous hydrogen sulfide compounds using microbial fuel cell.

In this study, a microbial fuel cell (MFC) was used to control malodorous hydrogen sulfide compounds generated from domestic wastewaters. The electricity production demonstrated a distinct pattern of a two-step increase during 170 h of system run: the first maximum current density was 118.6 ± 7.2 mA m⁻² followed by a rebound of current density increase, reaching the second maximum of 176.8 ± 9.4 mA m⁻². The behaviors of the redox potential and the sulfate level in the anode compartment indicated that the microbial production of hydrogen sulfide compounds was suppressed in the first stage, and the hydrogen sulfide compounds generated from the system were removed effectively as a result of their electrochemical oxidation, which contributed to the additional electricity production in the second stage. This was also directly supported by sulfur deposits formed on the anode surface, which was confirmed by analyses on those solids using a scanning electron microscope equipped with energy dispersive X-ray spectroscopy as well as an elemental analyzer. To this end, the overall reduction efficiencies for HS⁻ and H2S(g) were as high as 67.5 and 96.4 %, respectively. The correlations among current density, redox potential, and sulfate level supported the idea that the electricity signal generated in the MFC can be utilized as a potential indicator of malodor control for the domestic wastewater system.