Redox mediators are commonly used in microbial electrochemical systems to enable or enhance the electron transfer between microorganisms and electrodes. In recent studies, new insights into the mechanism of mediated extracellular electron transfer were gained, but some questions remain unanswered. In this review, some of the most outstanding research questions regarding the use of redox mediators in microbial electrochemical systems were discussed. These included the recycling of artificial and natural redox mediators, limitations in electron transfer rates by mediator turnover, metabolic burden, membrane permeability, and the putative interaction sites between commonly used redox mediators and the proteins of the electron transport chain of diverse electroactive microorganisms. To simplify the planning of mediator‐based bioelectrochemical systems, these molecular interaction sites were defined by their redox potential and are assigned to redox mediators, known or hypothesized to be able to transfer electrons from or to the specific interaction site. Furthermore, we addressed the kinetics of mediator transfer through the membrane and the potential rate‐limiting step in mediator‐based processes.

Bacterial cell appendix formation supports cell-cell interaction, cell adhesion and cell movement. Additionally, in bioelectrochemical systems (BES), cell appendages have been shown to participate in extracellular electron transfer. In this work, the cell appendix formation of Clostridium acetobutylicum in biofilms of a BES are imaged and compared with conventional biofilms. Under all observed conditions, the cells possess filamentous appendages with a higher number and density in the BES. Differences in the amount of extracellular polymeric substance in the biofilms of the electrodes lead to the conclusion that the cathode can be used as electron donor and the anode as electron acceptor by C. acetobutylicum. When using conductive atomic force microscopy, a current response of about 15 nA is found for the cell appendages from the BES. This is the first report of conductivity for clostridial cell appendices and represents the basis for further studies on their role for biofilm formation and electron transfer.

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Vibrio natriegens promises to be a new standard biotechnological working organism since it grows extraordinarily fast, its productivity surpasses E. coli by far, and genomic tools are getting readily available. Recent studies provided insights into its extracellular electron transfer pathway, revealing it to be similar to other well-known electroactive organisms. Therefore, we aimed to show for the first time that V. natriegens donates electrons from its metabolism to an electrode by direct contact as well as via an artificial redox mediator. Our results demonstrate current densities up to 196 μA cm À 2 using an artificial mediator. Via direct electron transfer, 6.6 μA cm À 2 were achieved within the first 24 h of cultivation. In the mediated system, mainly formate, acetate, and succinate were produced from glucose. These findings favor V. natriegens over established electroactive organisms due to its superior electrontransfer capabilities combined with an outstanding metabolism.