Electron Transfer via Reduced Metabolic Products

When fermentative microorganisms and yeast are used for MFCs operation, the microbial fermentative or photo-heterotrophic processes will produce certain energy-rich reduced metabolites, which can be abiotically oxidised at the anode surface to provide electron. These products might include hydrogen, alcohols, ammonia or formate. Ffowever, up to now only a few studies documented this mechanism or directly quantified which reduced products were oxidised at the anode [19,56]. With the low yield of such fermentation products, and the slow electrode reactions, these systems are inherently inefficient. Although it is possible to modify the composition of anodes to increase their reactivity with some metabolic end products, these electrodes tend to foul wifh oxidation products. 

One example of such reduced metabolic products that readily react with electrodes is Fl2S, which is produced by sulphate reducers in MFCs [19]. The primary oxidation product in this process is insoluble sulphur (S°). Reduction of sulphate to sulphide requires eight electrons, but oxidation of sulphide to S° releases only two of fhese eight electrons at the electrode surface. For example, when pyruvafe was used as the substrate, the above electrochemical reactions might be expressed as 

4 Summary Recent Advances in Understanding the Role of Microorganisms 

As discussed above, the exocellular electron transfer mechanisms in MFC systems are rather complex. Different microbes may transfer electrons via quite distinct approaches, and one specific microorganism may have more than one electron transfer paths. For example, Shewanella spp. have outer membrane C5tiochromes for direct electron transfer by contact, but they can also produce electrically conductive nanowires. S. oneidensis also produces flavins that can function as electron shuttles. MFCs are mostly inoculated 

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