Recent progress in the characterization and application of exo-electrogenic microorganisms.

Exo-electrogenic microorganisms are characterized by their special metabolic capability of transferring metabolic electrons out of their cell, into insoluble external electron acceptors such as iron or manganese oxides and electrodes, or vice versa take up electron from electrodes. Their conventional application is primarily limited to microbial fuel cells for electrical power generation and microbial electrolysis cells for the production of value-added products such as biohydrogen, biomethane and hydrogen peroxide. The utility of exo-electrogenic organisms has expanded into many other applications in recent times. Such examples include microbial desalination cells, microbial electro-synthesis cells producing value-added chemicals such as bio-butanol and their applications in other carbon sequestration technologies. Additionally, electrochemically-active organisms are now beginning to be employed in biosensor applications for environmental monitoring. Additionally, the utility of biocathodes in bio-electrochemical systems is also a novel application in catalyzing the cathodic oxygen reduction reaction to enhance their electrochemical performance. Advances have also been made in the expansion and use of other organisms such as the usage of photosynthetic microorganisms for the fabrication of self-sustained bio-electrochemical systems. This review attempts to provide a comprehensive picture of the state-of the art of exo-electrogenic organisms and their novel utility in bioelectrochemical systems.