Electron Discharge from the Biocatalyst

Microbes with high electron discharge capability are considered to be electro­chemically active and are crucial in the MFC operation. Electron transfer from the biocatalyst metabolic activities to the anode (intermediary electron acceptor) is catalyzed by two mechanisms, viz., direct electron transfer (DET) and mediated electron transfer (MET), based on the electron carrier involved (Fig. 14.2). DET is possible by means of physical contact of the bacterial cell with the anode and without the involvement of any redox species or mediator. Some of the membrane


Fig. 14.2 Schematic representation of electron transfer mechanism from microbial metabolism to the anode bound cell organelles viz., cytochromes, nanowires, etc., will help in the electron transfer from outer membrane of the bacterial cell to the external electron acceptor (anode). Geobacter, Rhodoferax, and Shewanella are the most-studied bacterial species for their exocellular electron transfer through membrane bound organelles (Kim et al. 1999; Chaudhuri and Lovley 2003; Holmes et al. 2004; Lovley 2006; Chang et al. 2006; Schroder 2007).

The name MET itself indicates the involvement of mediators for the exocellular electron transfer from the biocatalyst to the anode. The mediators may be artifi­cially added or naturally excreted soluble shuttlers or primary and secondary metabolites from bacterial metabolism (Schroder 2007). Both the natural and
artificial mediators are available for the electron transfer in a wide range, viz., inorganic compounds such as iron or magnesium containing metal complexes, organic compounds such as quinine and phenazine derivatives.

Microbes grown under electron acceptor depleted conditions and distant from the anode will also tend to release low molecular weight electron shuttling com­pounds through secondary metabolic pathways, e. g., pyocyanine and 2-amino-3- carboxy-1, 4-naphthoquinone (ACNQ), phenazine-1-carboxamide, etc., (Newman and Kolter 2000; Hernandez and Newman 2001; Newman 2001). Pseudomonas group of microbes secretes pyocyanin and pyovirdin like colored compounds for the electron shuttling. Phenazines, phenoxazines, quinines, etc., are the natural mediators investigated in the literature for their suitability as mediators (Bennetto et al. 1983; Roller et al. 1984; Bennetto 1990; Park and Zeikus 1999; Newman and Kolter 2000; Park and Zeikus 2000; Rabaey et al. 2004; Schroder 2007).

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