Performance of stacked microbial fuel cells with barley-shochu waste.

The treatment of barley-shochu waste combined with electricity generation was examined using stacked microbial fuel cells (SMFCs). The maximum chemical oxygen demand (CODCr) removal efficiency and maximum power density were achieved at 36.7 ± 1.1% and 4.3 ± 0.2 W m⁻³ (15.7 ± 0.9 mW m-2). The acetic acid concentration in effluent increased, whereas the citric acid, ethanol and sugar concentrations decreased during the operation. Microbial community analysis of the anode cell suspension and raw barley-shochu waste revealed that Clostridiaceae, Acetobacteraceae, and Enterobacteriaceae became predominant after the operation, implying that microorganisms belonging to these families might be involved in organic waste decomposition and electricity generation in the SMFCs.

Unexpected genomic features of high current density-producing Geobacter sulfurreducens strain YM18.

Geobacter sulfurreducens produces high current densities and it has been used as a model organism for extracellular electron transfer studies. Nine G. sulfurreducens strains were isolated from biofilms formed on an anode poised at -0.2 V (vs SHE) in a bioelectrochemical system in which river sediment was used as an inoculum. The maximum current density of an isolate, strain YM18 (9.29 A/m2), was higher than that of the strain PCA (5.72 A/m2), the type strain of G. sulfurreducens, and comparable to strain KN400 (8.38 A/m2), which is another high current-producing strain of G. sulfurreducens. Genomic comparison of strains PCA, KN400 and YM18 revealed that omcB, xapD, spc and ompJ, which are known to be important genes for iron reduction and current production in PCA, were not present in YM18. In the PCA and KN400 genomes, two and one region(s) encoding CRISPR/Cas systems were identified, respectively, but they were missing in the YM18 genome. These results indicate that there is genetic variation in the key components involved in extracellular electron transfer among G. sulfurreducens strains.

Proteolytic Maturation of the Outer Membrane c-Type Cytochrome OmcZ by a Subtilisin-Like Serine Protease Is Essential for Optimal Current Production by Geobacter sulfurreducens.

An outer membrane c-type cytochrome (OmcZ) in Geobacter sulfurreducens is essential for optimal current production in microbial fuel cells. OmcZ exists in two forms, small and large, designated OmcZS and OmcZL, respectively. However, it is still not known how these two structures are formed. A mutant with a disruption of the GSU2075 gene encoding a subtilisin-like serine protease (designated ozpA for the OmcZprotease), which is located downstream of omcZ, produced low currents at a level similar to that of the omcZ-deficient mutant strain. Biochemical analyses revealed that the ozpA mutant accumulated OmcZL and did not produce OmcZS, which is thought to be a mature form that is essential for the extracellular electron transfer to the electrode. A heterologous expression system cell lysate from an Escherichia coli strain producing OzpA cleaved OmcZL and generated OmcZS as the proteolytic product. Among the culture supernatant, loosely bound outer surface, and intracellular protein fractions from wild-type G. sulfurreducens, only the culture supernatant protein fraction showed OmcZL cleavage activity, indicating that the mature form of OmcZ, OmcZS, can be produced outside the cells. These results indicate that OzpA is an essential protease for current production via the maturation of OmcZ, and OmcZS is the key to the extracellular electron transfer to electrodes. This proteolytic maturation of OmcZ is a unique regulation among known c-type cytochromes in G. sulfurreducens. IMPORTANCE Microbial fuel cells are a promising technology for energy generation from various waste types. However, the molecular mechanisms of microbial extracellular electron transfer to the electrode need to be elucidated. G. sulfurreducens is a common key player in electricity generation in mixed-culture microbial fuel cell systems and a model microorganism for the study of extracellular electron transfer. Outer membrane c-type cytochrome OmcZ is essential for an optimal current production by G. sulfurreducens. OmcZ proteolytic cleavage occurs during maturation, but the underlying mechanism is unknown. This study identifies a subtilisin-like protease, OzpA, which plays a role in cleaving OmcZ and generating the mature form of OmcZ (OmcZS). OzpA is essential for current production and, thus, the proteolytic maturation of OmcZ. This is a novel regulation of the c-type cytochrome for G. sulfurreducens extracellular electron transfer. This study also provides new insights into the design strategy and development of microbial extracellular electron transfer for an efficient energy conversion from chemical energy to electricity.

Electricity generation from sweet potato-shochu waste using microbial fuel cells.

Electricity generation and treatment of sweet potato-shochu waste, acidic and organic-rich slurry, was examined using cassette-electrode microbial fuel cells (CE-MFCs). Among CE-MFCs with raw (73 g-chemical oxygen demand chromium CODCr/L) and different concentration of diluted sweet potato-shochu waste (0.5, 1, 5, 10, and 20 g-CODCr/L) without pH control, the maximum power density (1.2 W/m3) and CODCr removal efficiency (67.4 ± 1.8%) were observed in the CE-MFCs with 10 g-CODCr/L shochu waste. The concentration of organic acid was decreased to below the quantification limits during the 9-day operation in the CE-MFC with 10 g-CODCr/L shochu waste. During the same period, the electrolyte pH was increased from 4.2 to 6.6. Microbial community analysis revealed that the genus Clostridium (75.4%) was predominant in the CE-MFCs with raw shochu waste, whereas Bacteroides (65.3%) and Clostridium (12.1%) were predominant in the CE-MFCs with 10 g-CODCr/L shochu waste.