Direct electron transfermechanism

Mediator-less microbial fuel cells do not require a mediator but uses electrochemically active bacteria to transfer electrons to the electrode (electrons are carried directly from the bacterial respiratory enzyme to the electrode) (Kim et al., 1999; Chaudhuri and Lovely, 2003).

Mediator-less microbial fuel cells, on the other hand, use special microbes which possess the ability to donate electrons to the anode provided oxygen (a stronger electrophilic agent) is absent (Scholz et al, 2003; Mohan et al, 2007). There are variants of the mediatorless microbial fuel cell which differ with respect to the sources of nutrient and type of inoculum used. They are the Plant-Microbial Fuel Cell, the Soil-Microbial Fuel Cell and the Metal Reducing-Microbial Fuel Cell and the Sediment Microbial Fuel Cell.

There are several microorganisms (eg. Shewanella putrefaciens, Geobactersulferreducens, G.metallireducens and Rhodoferax ferrireducens) that transfer electrons frominside the cell to extracellular acceptors via c-type cytochromes, biofilms and highly conductivepili (nanowires) (Derek R, 2008). These microorganisms have high coulombic efficiency andcan form biofilms on the anode surface that act as electron acceptors and transfer electronsdirectly to the anode resulting in the production of more energy (Chaudhuri and Lovley, 2003;Kim et al., 2002).

Direct transfer via outer-membrane cytochromes and transfer via conductive pili, also known as ‘nanowires’ (e.g. in Geobacter species and Aeromonas hydrophila) (Bond et al. 2003; Lovely 2006; Rabaey et al. 2004; Torres et al. 2009).

Nanowires

Transfer of electrons via pili has been known for some time. Tests involving Geobacter sulfurreducens with pili and genetic mutants deficient in pili confirmed that the pili were highly conductive; suggesting that electron transfer through these biological nanowires is responsible for the reduction of Fe (III) oxidesin this species (Regueraet al., 2005). There is some debate ongoing about the conductivity and role of pili in electrontransfer and recent works have suggested that soluble riboavins in the supernatant may also play aprominent role in electron transfer in Shewanella sp. (Marsili et al., 2008).

Gorby and co-workers reported the occurrence of conductive appendages for both Geobacter and Shewanella species (Gorby and Beveridge 2005) which were termed bacterial “nanowires” . The conductivity of the appendages was examined using conductive scanning tunneling microscopy (STM) where a sample is loaded onto a highly ordered pyrolytic graphite surface (a very flat and conductive surface), and a conductive (Pt-Ir) tip is rastered across the sample under constant-current imaging conditions. The resulting current-voltage curves demonstrate the portion of the scan that is conductive relative to the surface (Bruce E. Logan 2008).

Cell- surface electron transfer

Close examination of the previous micrograph shows that there are surface blebs, i.e., protrusions on the surface that do not exist as nanowires but certainly could be conductive points of contact. Still, the small proteins responsible for electron transfer from the cell surface might not even be visible in such micrographs. Anaerobically grown Shewanella oneidensis adhered to an iron (goethite) surface with two to five times greater force than aerobically grown cells, and so the observation that this strain was more adhesive under anaerobic conditions might allow closer contact required for electron transfer from cell bound cytochromes even in the absence of nanowires (Lower et al. 2001).

Thank you very much for the insight Abdo Ahmad

Those all look as great references. I wish I could see the complete tittle of such publications.

its my pleasure