Microbial direct electron transfer

Whereas the mediated electron transfer, for example, via the oxidation of primary metabolites, can be realized with cell suspensions (see later) without any direct interaction of the living cell and the electrode material, an attachment of the bacterial cells (or its appendices) to an electrode surface is compulsory for the direct electron transfer. This attachment is not necessarily permanent, some bacterial species possess a directed motility toward prospective electron acceptors such as electrodes allowing a DET of suspended cells [24]. Furthermore, there is growing evidence that the matrix of exopolymeric substances (EPS) may also play an important role in the microbial electron transfer [25]. 

This chapter provides examples of various biocatalysts, nanomaterials, and fabrication processes that yield functional bioelectrodes for anodic or cathodic processes. Most of the descriptions of electrode materials in this chapter focus on the fabrication of electrode architectures suitable for direct electron transfer (DET) processes, with an emphasis on enzyme-based electrodes however, examples of materials that are also suitable for microbial anodes are also included because of the parallels in development of such conductive architectures (see Sections 10.4.2 and 10.5.2). 

From Eq. (1.42), it can be inferred that the substrate breaks down into carbon dioxide and proton along with electricity as a by-product. The concept of MFC was demonstrated by Potter in 1910, who used platinum electrodes as well as living cultures of Escherichia coli and Saccharomyces. The anodophile species of the microbes can transfer the electrons directly to the anode. Otherwise, the electron mediators are required in the cell for enhanced power output and increased efficiency. The direct electron transfer to the anode is hindered by a majority of microbial species due to the presence of non-conductive Upopolysaccharides, peptididoglycans and lipid membrane in their outer layers. Hence, mediators are used which capture electrons from the membrane and are reduced. Furthermore, these mediators will again become oxidised once they move across the membrane and release the electrons to the anode. Hence, the electron transfer process keeps the anode replenished which maintains the sustainability of cell. Anthraquinone-2, 6-disulphonate, 2-methylene blue, thionine, 2-hydroxy-l, 4-naphthoquinone, Fe (III) EDTA, Meldola s blue and neutral red are some of the common chemical mediators which enhance electricity production. MFC is known as the... 

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. 

D. Prasad, S. Arun, M. Murugesan, S. Padmanaban, R.S. Satyanarayanan, S. Berchmans and V. Yegnaraman, Direct electron transfer with yeast cells and construction of a mediatorless microbial fuel ceU, Biosens. Bioelectron. 22, 2007, 2604-2610. 

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). 

Like the various forms of iron, NOM apparently serves as both bulk reductant and mediator of reduction as well as bulk reductant (recall section 2.2.2). NOM also can act as an electron acceptor for microbial respiration by iron reducing bacteria (26), thereby facilitating the catabolism of aromatic hydrocarbons under anaerobic conditions (103). In general, it appears that NOM can mediate electron transfer between a wide range of donors and acceptors in environmental systems (104,105). In this way, NOM probably facilitates many redox reactions that are favorable in a thermodynamic sense but do not occur by direct interaction between donor and acceptor due to unfavorable kinetics. 

In particular, redox chemicals (serving as electron shuttles) naturally synthesized by bacteria or exogenously added synthetic molecules have been proved to be directly involved in promoting extracellular electron transfer between the cells and the electrode. Moreover, electrode modification with conductive polymers or carbon nanomaterials showed great potential for the enhancement of nanoscale topological interactions and hence the extracellular electron transfer between the cells and the electrode.Extracellular electron transfer manipulation (a microbial process) with chemical electron shuttles or electrode modifiers can be considered as a typical application of chemical bioengineering. 

Purely electrochemical or microscopic techniques (see below) only allow a limited insight into the microbial electron transfer processes. Consequently, the use of spectroelectrochemical techniques, i.e., the direct coupling electrochemical and spectroscopic analysis, has been demonstrated. These techniques, including for instance the combination of voltammetry and UV/vis spectroscopy [19], attenuated total reflectance surface-enhanced infrared absorption spectroscopy... 

Figure 9.2. Mechanisms of EET (a) Electron transfer through use of redox mediator. Mediator is reduced by bacterium and subsequently oxidized by electrode. Use of mediator to transfer electrons to other bacteria may be possible, (b) Electron transfer through outer membrane c-type cytochromes. Electrons are transferred through outer membrane proteins in direct contact with electrode, (c) Electron transfer through use of microbial nano-wires. Conductive pili confer conductivity to the bio film allowing electron transfer to other bacteria as well as electrode transfer of electrons from nano-wires to electrodes likely catalyzed by c-type cytochromes dispersed in the biofilm.

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