Influence of Anode Potentials on Current Generation and Extracellular Electron Transfer Paths of Geobacter Species (original) (raw)
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In this study the characterization of Geoalkalibacter subterraneus is presented, which is a novel halophilic anode respiring bacterium (ARB) previously selected and identified in a potentiostatically controlled bioelectrochemical system (BES) inoculated with sediments from a salt plant. Pure culture electroactive biofilms of Glk. subterraneus were grown during chronoamperometric batch experiments at a graphite electrode poised at +200 mV (vs. SCE) with 10 mM acetate as the electron donor. These biofilms exhibited the highest current density (4.68 ± 0.54 A m−2) reported on a planar material with a pure culture under saline conditions (3.5% NaCl). To investigate possible anodic electron transfer (ET) mechanisms, cyclic voltammetry (CV) of mature visible apparent reddish biofilms was performed under bioelectrocatalytic substrate consumption (turnover) and in the absence of the substrate (non-turnover). CV evidenced a well defined typical sigmoidal shape and a pair of clear redox couples under turnover and non-turnover conditions, respectively. Moreover, the calculation of their formal potentials indicated the presence of a common ET mechanism present in both CV conditions between −427.6 ± 0.5 (Ef,2) and −364.8 ± 4.5 mV (Ef,3). Confocal laser scanning microscopy inspection showed a biofilm structure composed of several layers of metabolically active bacteria that spread all over the electrode material within a biofilm up to 76 ± 7 μm thick. Such high value compared to the thickness values normally reported in the literature for pure culture electroactive bacteria justifies further investigations. Taken together, these results suggest that Glk. subterraneus performs a direct ET mechanism in contact with the electrode material. Furthermore, direct current generation from saline wastewater significantly expands the application of BESs.
Energy & Environmental Science, 2009
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.
Energy Environ. Sci., 2011
A biofilm of Geobacter sulfurreducens will grow on an anode surface and catalyze the generation of an electrical current by oxidizing acetate and utilizing the anode as its metabolic terminal electron acceptor. Here we report qualitative analysis of cyclic voltammetry of anodes modified with biofilms of G. sulfurreducens strains DL1 and KN400 to predict possible rate-limiting steps in current generation. Strain KN400 generates approximately 2 to 8-fold greater current than strain DL1 depending upon the electrode material, enabling comparative electrochemical analysis to study the mechanism of current generation. This analysis is based on our recently reported electrochemical model for biofilm-catalyzed current generation expanded here to a five step model; Step 1 is mass transport of acetate, carbon dioxide and protons into and out of the biofilm, Step 2 is microbial turnover of acetate to carbon dioxide and protons, Step 3 is the non-concerted, 1-electron reduction of 8 equivalents of electron transfer (ET) mediator, Step 4 is extracellular electron transfer (EET) through the biofilm to the electrode surface, and Step 5 is the reversible oxidation of reduced mediator by the electrode. Five idealized voltammetric current vs. potential dependencies (voltammograms) are derived, one for when each step in the model is assumed to limit catalytic current. Comparison to experimental voltammetry of DL1 and KN400 biofilm-modified anodes suggests that for both strains, the microbial oxidation of acetate (Step 2) is fast compared to microbial reduction of ET mediator (Step 3), and either Step 3 or EET through the biofilm (Step 4) limits catalytic current generation. The possible limitation of catalytic current by Step 4 is consistent with proton concentration gradients observed within these biofilms and finite thicknesses achieved by these biofilms. The model presented here has been universally designed for application to biofilms other than G. sulfurreducens and could serve as a platform for future quantitative voltammetric analysis of non-corrosive anode and cathode reactions catalyzed by microorganisms.
Frontiers in Microbiology, 2016
Studies on the mechanisms for extracellular electron transfer in Geobacter species have primarily focused on Geobacter sulfurreducens, but the poor conservation of genes for some electron transfer components within the Geobacter genus suggests that there may be a diversity of extracellular electron transport strategies among Geobacter species. Examination of the gene sequences for PilA, the type IV pilus monomer, in Geobacter species revealed that the PilA sequence of Geobacter uraniireducens was much longer than that of G. sulfurreducens. This is of interest because it has been proposed that the relatively short PilA sequence of G. sulfurreducens is an important feature conferring conductivity to G. sulfurreducens pili. In order to investigate the properties of the G. uraniireducens pili in more detail, a strain of G. sulfurreducens that expressed pili comprised the PilA of G. uraniireducens was constructed. This strain, designated strain GUP, produced abundant pili, but generated low current densities and reduced Fe(III) very poorly. At pH 7, the conductivity of the G. uraniireducens pili was 3 × 10 −4 S/cm, much lower than the previously reported 5 × 10 −2 S/cm conductivity of G. sulfurreducens pili at the same pH. Consideration of the likely voltage difference across pili during Fe(III) oxide reduction suggested that G. sulfurreducens pili can readily accommodate maximum reported rates of respiration, but that G. uraniireducens pili are not sufficiently conductive to be an effective mediator of long-range electron transfer. In contrast to G. sulfurreducens and G. metallireducens, which require direct contact with Fe(III) oxides in order to reduce them, G. uraniireducens reduced Fe(III) oxides occluded within microporous beads, demonstrating that G. uraniireducens produces a soluble electron shuttle to facilitate Fe(III) oxide reduction. The results demonstrate that Geobacter species may differ substantially in their mechanisms for long-range electron transport and that it is important to have information beyond a phylogenetic affiliation in order to make conclusions about the mechanisms by which Geobacter species are transferring electrons to extracellular electron acceptors.
Electricity production by Geobacter sulfurreducens attached to electrodes
2003
ABSTRACT Previous studies have suggested that members of the Geobacteraceae can use electrodes as electron acceptors for anaerobic respiration. In order to better understand this electron transfer process for energy production, Geobacter sulfurreducens was inoculated into chambers in which a graphite electrode served as the sole electron acceptor and acetate or hydrogen was the electron donor.
Microarray and genetic analysis of electron transfer to electrodes in Geobacter sulfurreducens
Environmental Microbiology, 2006
Whole-genome analysis of gene expression in Geobacter sulfurreducens revealed 474 genes with transcript levels that were significantly different during growth with an electrode as the sole electron acceptor versus growth on Fe(III) citrate. The greatest response was a more than 19-fold increase in transcript levels for omcS , which encodes an outer-membrane cytochrome previously shown to be required for Fe(III) oxide reduction. Quantitative reverse transcription polymerase chain reaction and Northern analyses confirmed the higher levels of omcS transcripts, which increased as power production increased. Deletion of omcS inhibited current production that was restored when omcS was expressed in trans. Transcript expression and genetic analysis suggested that OmcE, another outer-membrane cytochrome, is also involved in electron transfer to electrodes. Surprisingly, genes for other proteins known to be important in Fe(III) reduction such as the outer-membrane c -type cytochrome, OmcB, and the electrically conductive pilin 'nanowires' did not have higher transcript levels on electrodes, and deletion of the relevant genes did not inhibit power production. Changes in the transcriptome suggested that cells growing on electrodes were subjected to less oxidative stress than cells growing on Fe(III) citrate and that a number of genes annotated as encoding metal efflux proteins or proteins of unknown function may be important for growth on electrodes. These results demonstrate for the first time that it is possible to evaluate gene expression, and hence the metabolic state, of microorganisms growing on electrodes on a genome-wide basis and suggest that OmcS, and to a lesser extent OmcE, are important in electron transfer to electrodes. This has important implications for the design of electrode materials and the genetic engineering of microorganisms to improve the function of microbial fuel cells.
Environmental Science & Technology, 2008
The mechanism(s) by which electricity-producing microorganisms interact with an electrode is poorly understood. Outer membrane cytochromes and conductive pili are being considered as possible players, but the available information does not concur to a consensus mechanism yet. In this work we demonstrate that Geobacter sulfurreducens cells are able to change the way in which they exchange electrons with an electrode as a response to changes in the applied electrode potential. After several hours of polarization at 0.1 V Ag/AgClsKCl (saturated), the voltammetric signature of the attached cells showed a single redox pair with a formal redox potential of about -0.08 V as calculated from chronopotentiometric analysis. A similar signal was obtained from cells adapted to 0.4 V. However, new redox couples were detected after conditioning at 0.6 V. A large oxidation process beyond 0.5 V transferring a higher current than that obtained at 0.1 V was found to be associated with two reduction waves at 0.23 and 0.50 V. The apparent equilibrium potential of these new processes was estimated to be at about 0.48 V from programmed current potentiometric results. Importantly, when polarization was lowered again to 0.1 V for 18 additional hours, the signals obtained at 0.6 V were found to greatly diminish in amplitude, whereas those previously found at the lower conditioning potential were recovered. Results clearly show the reversibility of cell adaptation to the electrode potential and point to the polarization potential as a key variable to optimize energy production from an electricity producing population.