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Nitrogenases are the only enzymes known to reduce molecular nitrogen (N ) to ammonia (NH ). By using methyl viologen (N,N'-dimethyl-4,4'-bipyridinium) to shuttle electrons to nitrogenase, N reduction to NH can be mediated at an electrode surface. The coupling of this nitrogenase cathode with a bioanode that utilizes the enzyme hydrogenase to oxidize molecular hydrogen (H ) results in an enzymatic fuel cell (EFC) that is able to produce NH from H and N while simultaneously producing an electrical current. To demonstrate this, a charge of 60 mC was passed across H /N EFCs, which resulted in the formation of 286 nmol NH mg MoFe protein, corresponding to a Faradaic efficiency of 26.4 %.

Microbial-electrode electron transfer is a mechanism by which microbes make their living coupling to electronic circuits, even across long distances. From a chemistry perspective, it represents a model platform that integrates biological metabolism with artificial electronics, and will facilitate the fundamental understanding of charge transport properties within these distinct chemical systems and particularly at their interfaces. From a broad standpoint, this understanding will also open up new possibilities in a wide range of high impact applications in bioelectrochemical system based technologies, which have shown promise in electricity, biochemical, chemical feedstock production but still require many orders of magnitude improvement to lead to viable technologies. Here we review opportunities to understand microbial-electrode electron transfer to improve electrocatalysis (bioelectricity) and electrosynthesis (biochemical and chemical production). We discuss challenges and the ample interdisciplinary research opportunities and suggest paths to take to improve production of fuels and chemicals at high yield and efficiency and the new applications that may result from increased understanding of the microbial-electrode electron transfer mechanism.Bio-electrochemical system (BES) can be expressed as the bidirectional electron transports between biotic and abiotic components, where the redoxactive microorganisms or bio-macromolecules act as the catalysts that facilitate the exchange process 1 . A glossary of important terms is provided in box 1. A model system of BES that has been widely studies is the Microbial Fuel Cell (MFCs). Similar to the conventional fuel cell, the microorganisms can transport electrons to the anodes of MFC after oxidizing the electron donors, thus generating the electrical flow toward the cathode 2 . Meanwhile, certain microorganisms are also known for their capability to reduce the electron acceptors such as nitrate, perchlorate or metals in the cathodes 3 . Other BESs such as Microbial electrolysis cells (MEC), Microbial electrosynthesis (MES),Microbial solar cells (MSCs), and Plant microbial fuel cells (PMFCs) also share similar electron transport strategy. These direct electron transport processes created a novel and promising possibility to bridge the fundamental researches in microbiology, electrochemistry, environmental engineering, material science and the applications in waste remediation & resource recovery, sustainable energy production, and bio-inspired material development. The basic working principles and the applications of these different BESs have been comprehensively reviewed by many different groups [4][5][6][7] . Bioelectrochemcial systemsEnzymatic electron transport process is one of the earliest BES models which received extensive attention due to the interests in development of amperometric biosensors and enzymatic fuel cell in late 20 th century [8][9][10][11][12] . In this system, the electrons generated from specific enzymatic reactions can be either...

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