Extracellular electron transfer

Nevin KP, Lovley DR (2002b) Mechanisms for accessing insoluble Fe(III) oxide during dissimilatory Fe(III) reduction by Geothrixfermentans. App Environ Microbio 68 2294-2299 Newman DK, Kolter R (2000) A role for excreted quinones in extracellular electron transfer. Nature 405 94-97... 

Newman D. K. and Kolter R. (2000) A role for excreted quinones in extracellular electron transfer. Nature 405, 94-97. 

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. 

As the cell membrane of microorganisms is non-conductive, how to transfer the electrons across the cell membrane (usually termed as extracellular electron transfer (EET)) is the key issue for MES. Eor the bioanode, extracellular electron transfer usually refers to outwards electron transfer, which is related to transportation of intracellular electrons to the solid electrode. To date, at least three electron transfer pathways have been explored, i.e., electron shuttle mediated electron transfer, outer membrane redox proteins mediated contact-based electron transfer, and conductive pili mediated electron transfer (Figure 5.5). ... 

Figure 5.5 Simple model for extracellular electron transfer between cells and an electrode. SHue electron shuttles in redox state. SHqx electron shuttles in oxide state.

High electron discharge capabilities of electrochemically active microbes have a significant impact on the performance of the corresponding MES. To date, three kinds of mechanisms for extracellular electron transfer (EET) involved in MES have been established ... 

In addition to self-generated shuttles, synthetic compounds with similar chemical structures also show the same function. Changing the molecular structure of phenazine-type redox mediators by artidcial synthesis can signid-cantiy induence microbial extracellular electron transfer. Immobilization... 

Microbial electrocatalysis relies on microorganisms as catalysts for reactions occurring at electrodes. The microorganisms involved are able to transport electrons in and out of the cell, a process known as extracellular electron transfer (EET), and can catalyze both oxidation and reduction reactions [80, 81]. Their catalytic properties have been confirmed by the fact that they are able to lower the overpotentials (lower energy loss) at both anodes [82] and cathodes [56, 69], giving an increased performance of the system. Nevertheless, they cannot be considered as true catalysts since part of the substrate/electron donor is consumed for growth. 

Hofte, M., Verstraete, W., and Rabaey, K. (2008) Metabolites produced by Pseudomonas sp. enable a Gram-positive bacterium to achieve extracellular electron transfer. Appl. Microbiol. Biotechnol, 77 (5), 1119-1129. 

D. R. (2008) Shewanetta secretes flavins that mediate extracellular electron transfer. Proc. Natl. Acad. Sci. U. S. A., 105 (10), 3968-3973. 

Richter, H., Nevin, K.P., Jia, H.F., Lowy, D.A., Lovley, D.R., and Tender, L.M. (2009) Cydic 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. Energy Environ. Sci., 2 (5), 506—516. 

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. 

ELECTROCHEMICAL TECHNIQUES FOR STUDYING EXTRACELLULAR ELECTRON TRANSFER OF ELECTROCHEMICALLY ACTIVE BIOFILMS... 

The majority of this book is dedicated to describing how to study extracellular electron transfer of EABs. Once the correct reactor configuration has been chosen and the EAB has been successfiiUy grown or acclimatized on an electrode, the next step is to study the electron transfer properties of the biofilm electrode using electrochemical techniques. There are many electrochemical techniques available however, we introduce only those frequently used to study electron transfer processes in EAB literature. 

Only recently, however, has spectroelectrochemistry been used for in situ measurements of the redox state of cytochromes in thick, pregrown EABs in which extracellular electron transfer through the biofilm matrix was studied. It was shown that the c-type cytochromes inside thick G. sulfurreducens biofilms probed under nonturnover conditions were completely reduced at polarization potentials below -350 mVsHE and completely oxidized at potentials above -ElOO mVsHE> demonstrating long-range extracellular electron transfer through the cytochrome network [66]. 

Translation of electrochemically active biqfilm extracellular electron transfer research to bioelectrochemical systems... 

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