Cyclic voltammetry of biofilms of wild type and mutant Geobacter sulfurreducens on fuel cell anodes indicates possible roles of OmcB, OmcZ, type IV pili, and protons in extracellular electron transfer

Abstract

Geobacteracea are distinct for their ability to reduce insoluble oxidants including minerals and electrodes without apparent reliance on soluble extracellular electron transfer (ET) mediators. This property makes them important anode catalysts in new generation microbial fuel cells (MFCs) because it obviates the need to replenish ET mediators otherwise necessary to sustain power. Here we report cyclic voltammetry (CV) of biofilms of wild type (WT) and mutant G. sulfurreducens strains grown on graphite cloth anodes acting as electron acceptors with acetate as the electron donor. Our analysis indicates that WT biofilms contain a conductive network of bound ET mediators in which OmcZ (outer membrane c-type cytochrome Z) participates in homogeneous ET (through the biofilm bulk) while OmcB mediates heterogeneous ET (across the biofilm/electrode interface); that type IV pili are important in both reactions; that OmcS plays a secondary role in homogenous ET; that OmcE, important in Fe(III) oxide reduction, is not involved in either reaction; that catalytic current is limited overall by the rate of microbial uptake of acetate; that protons generated from acetate oxidation act as charge compensating ions in homogenous ET; and that homogenous ET, when accelerated by fast voltammetric scan rates, is limited by diffusion of protons within the biofilm. These results provide the first direct electrochemical evidence substantiating utilization of bound ET mediators by Geobacter biofilms and the distinct roles of OmcB and OmcZ in the extracellular ET properties of anode-reducing G. sulfurreducens.

Microtoming coupled to microarray analysis to evaluate the spatial metabolic status of Geobacter sulfurreducens biofilms

“…OmcZ is an outer-surface, c-type cytochrome that is essential for high-density current production and the OmcZ gene is more highly expressed in currentproducing cells Richter et al, 2009). Expression of omcZ was similar in the inner and outer biofilm.…”

Proteins involved in electron transfer to Fe(III) and Mn(IV) oxides by Geobacter sulfurreducens and Geobacter uraniireducens

“…Gene deletion studies have suggested that the outer membrane surface c-type cytochrome OmcZ (GSU2076) is important for growth on current-harvesting anodes (Inoue et al, 2010Nevin et al, 2009;Richter et al, 2009). omcZ was upregulated during growth on Mn(IV) oxide but not Fe(III) oxide (Table 4).…”

“…For example, whole-genome microarray comparisons of gene transcript abundance in current-producing biofilms versus biofilms reducing fumarate revealed that the octahaem c-type cytochrome OmcZ was one of the most highly expressed genes in current-producing biofilms . Subsequent gene deletion studies demonstrated that OmcZ was required for effective current production Richter et al, 2009) and further characterization demonstrated that OmcZ is an outer surface protein that is specifically localized at the biofilm-anode interface in current-producing biofilms (Inoue et al, 2010. In a previous study, Fe(III) reduction pathways were investigated by comparing gene expression in G. sulfurreducens growing with Fe(III) chelated with citrate as the electron acceptor and cells growing by fumarate reduction .…”

Biofilm conductivity is a decisive variable for high-current-density Geobacter sulfurreducens microbial fuel cells

“…This area calculation was performed with a confocal microscope by measuring the diameter and the thread count of the carbon fibers. 10 Since the cathode area is 275 times larger than anode, the cathode reaction is no longer a limiting factor. Fuel cell current density was calculated by dividing the steady-state current by the area of the graphite rod.…”

“…15 Furthermore, modeling of similar current-producing biofilms demonstrated that it was necessary to invoke biofilm conductivity in order to describe current production. 10,[16][17][18][19][20][21] Geobacter species were important components of all these systems.…”