Microbial electron transfer mechanisms

Biofuel cells — Figure. Schematic illustration of identified electron transfer mechanisms in microbial fuel cells. Electron transfer via (a) cell membrane-bound cytochromes, (b) electrically conductive pili (nanowires), (c) microbial redox mediators, and (d) via oxidation of reduced secondary metabolites [v]... 

Villano, M., and Angenent, LT, (2011) Cathodes as electron donors for microbial metabolism which extracellular electron transfer mechanisms are involved Bioresour. Technol, 102 (1), 324-333. 

Huang, LP., Regan, ).M., and Quan, X. (2011) Electron transfer mechanisms, new applications, and performance of biocathode microbial fuel cells. Bioresour. Technol., 102 (1), 316-323. 

Schroder U. Anodic electron transfer mechanisms in microbial fuel cells and their energy efficiency. Phys Chem ChemPhys 2007 9 2619-2629. 

Nielsen ME, Wu DM, Girguis PR, Reimers CE. Influence of substrate on electron transfer mechanisms in chambered benthic microbial fuel cells. Environ Sci Technol 2008 43 8671-8677. 

As the electron exchange with its substratum is an inevitable prerequisite for electroactive microbial biofilms the investigation of the underlying electron transfer mechanism calls for electrochemical techniques and methods. In the recent years several techniques have been exploited, most prominently... 

First we will discuss the similarities and differences between conventional chemical and microbial bioelectrocatalysts. Subsequently, the principal mechanisms of the extracellular electron transfer (EET) and the technology aiming at their exploitation will be introduced. Finally, the biological and electrochemical methods that allow the analysis of the microbial electron transfer and identification of the respective mediators, proteins, and microorganisms will be addressed. 

This chapter introduces the principles and applications of MFCs, with emphases on the nature of electricity-producing bacteria, anodic electron transfer mechanisms, power generation of MFCs and efficiency of electrode materials. Different types of MFCs and other microbial-electrochemical conversion devices are also discussed. 

Schematic diagram of the energy flux in anMFC. (Reprinted with permission from U. Schrdder, Anodic electron transfer mechanisms in microbial fuel cells and their energy efficiency, Phys. Chem. Chem. Phys. 9, 2007, 2619-2629. Copyright 2007 The Royal Society of Chemistry.)...

Various mechanisms of electron transfer and linking species have been identified and exploited. Basically there are two major types of linking species (i) soluble compounds (artificial or self-produced mediators), and (ii) compounds bonded to the microbial cell membrane (membrane-bond proteins or nanowires). Accordingly, electron transfer mechanisms from microorganisms to the electrode can be divided into five primary types, which will be discussed in the following section (1) direct cell-surface electron transfer, (2) direct electron transfer via nanowires, (3) electron transfer via exogenous redox mediators, (4) endogenous redox mediators and (5) reduced metabolic products. 

NO 3-Reducing. Fig. 9.15 shows data on groundwater below agricultural areas. The sharp decrease of 02 and NO3 at the redox cline indicate that the kinetics of the reduction processes are fast compared to the downward water transport rate. Postma et al., 1991 suggest that pyrite, present in small amounts is the main electron donor for NO3 reduction (note the increase of SOJ immediately below the oxic anoxic boundary). Since NO3 cannot kinetically interact sufficiently fast with pyrite a more involved mechanism must mediate the electron transfer. Based on the mechanism for pyrite oxidation discussed in Chapter 9.4 one could postulate a pyrite oxidation by Fe(III) that forms surface complexes with the disulfide of the pyrite (Fig. 9.1, formula VI) subsequent to the oxidation of the pyrite, the Fe(II) formed is oxidized direct or indirect (microbial mediation) by NO3. For the role of Fe(II)/Fe(III) as a redox buffer in groundwater see Grenthe et al. (1992). 

The first section deals specifically with the spectroscopic/ microscopic tools that can be used in concert with macroscopic techniques. The second section emphasizes computer models that are used to elucidate surface mediated reaction mechanisms. The remainder of the volume is organized around reaction type. Sections are included on sorption/desorption of inorganic species sorption/desorption of organic species precipitation/dissolution processes heterogeneous electron transfer reactions photochemically driven reactions and microbially mediated reactions. What follows are a few highlights taken from the work presented in this volume. 

Another promising approach for the detoxification of PCBs is the finding that anaerobic bacteria dechlorinate PCBs reductively [79, 80]. The authors used anaerobic microorganisms from Hudson River sediment and report that, at PCB concentrations of 700 ppm Aroclor, 63 per cent of the total chlorine was removed in 16 weeks, and the proportion of mono- and dichlorobiphenyls increased from 9 to 88 per cent. Dechlorination occurred primarily from the meta and para positions. These results indicate that reductive dechlorination may be an important environmental fate of PCBs, and suggest that a sequential anaerobic-aerobic biological treatment system for PCBs may be feasible. The proton source for the microbial reductive dechlorination of 2,3,4,5,6-pentachlorobiphenyl has been identified by Nies and Vogel [81]. Tlie authors report that the exact mechanism of the electron transfer for the dechlorination of PCBs is imknown however, they could show that the sotirce of tiie hydrogen atom is the proton from water, and that chloride is released from the PCB. 

Proton-coupled electron transfer (PCET) is known to play an important role in a variety of biological processes, including microbial iron transport by ferric enterobactin, enzyme catalysis in systems such as fumarate reductase and nitrate reducatase, and dioxygen binding by the non-heme iron protein hemerythrin. " As such, pH-dependent electrochemical studies can play an important role in unraveling these mechanisms. The most heavily studied biological system known to involve PCET is cytochrome c oxidase, the terminal electron-transfer complex of the mitochondrial respiratory chain, which catalyzes the reduction of molecular oxygen to water. ... 

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