Models for extracellular electron transport mechanisms coupled and uncoupled to energy conservation. (A) Schematic image of two different electron transport mechanisms between cathodes and the microbial metabolism analysed within this study. Cat1) Electron transport via a mediator and a cascade of membrane bound complexes (e.g. cytochromes) with simultaneous ATP generation; Cat2) Direct reduction of NAD to NADH by electrons and protons by membrane bound enzymes (e.g. hydrogenases) or diffusion of the mediator molecule. (B) Two different models for microbial interaction with an anode as electron sink. An1) Electrons from the quinone pool are transferred to the electron mediator by membrane-bound enzymes such as NADH-Ubiquinone oxidoreductase. As these complexes are proton pumping the created gradient can be used for ATP generation. An2) Electrons and protons are transferred simultaneously without creating a membrane potential.