## 1 Introduction  
Microbial fuel cells (MFCs) are electrochemical devices that utilize anaerobic electroactive microorganisms to promote generation of electricity as a renewable clean source of energy directly from wastewater, and recently, for bioremediation of contaminated sites [1, 2]. The working principle of MFCs is based on the activity of an electrochemically active biofilm that anaerobically oxidizes various organic substrates on the anode surface. The microorganisms that grow on the anode and that are responsible for the electron transfer are usually dissimilatory metal-reducing bacteria that are found ubiquitously in soils due to their ability to reduce insoluble elements, such as iron Fe\({}^{+3}\) and manganese Mn\({}^{+4}\) oxides [3]. The two best characterized families of dissimilatory metal reducing microorganisms in MFC systems are _Geobacteraceae_ and _Shewanellaceae_. They differ considerably in metabolic flexibility [4] and in their individual electron transfer mechanisms [5]. Additional species belonging to the _Desulfuronondaceae_ family are well recognized, yet less studied metal reducers. _Desulfurononas_ members reduce elemental sulfur to sulfide in order to obtain energy, but can also use fumarate, ferric iron (Fe\({}^{+3}\)), or manganese (Mn\({}^{+4}\)) as terminal electron acceptors [6]. Moreover, they can obtain energy from the complete oxidation of organic compounds to carbon dioxide, while reducing extracellular electron acceptors [7]. Therefore, it is not surprising that _Desulfuromonas_ and _Geobacter_ species are so closely related [8] and are often reported to be the most predominate taxa found on MFC anodes. Anodic biofilms may face different, often suboptimal and varying abiotic conditions during their formation and operation, which is a challenge for the operation of MFCs as a cost-effective wastewater treatment technology. Of particular concern is the necessity of real-world MFCs to operate incessantly, as a wastewater plant cannot be shut down in the case of MFC failure. Thus, there is a true need for ways to monitor MFC operation in real time to predict system failure, as well as a means to rapidly restart the system, should such failure occur. Major factors that can limit the electrical output in scaled-up MFCs are directly related to the electroactive microbial community, as well as its electron transport efficiency towards the anode. The anodic biofilm microbial composition has been considered to be a vital component of the MFC performance, and an important parameter for assessing the performance of MFCs and the metabolic processes taking place under different conditions [9; 10; 11; 12; 13; 14; 15]. Anodic biofilm enrichment involves specific selection processes that eventually dictate the biofilm community composition at steady state, i.e., when a stable maximal voltage has been obtained. Little is known about the microbial processes that actually stimulate the complex development of an anodic biofilm composition and function. Furthermore, anodic biofilms are often inappropriately considered to have reached the mature phase when a stable voltage output is first obtained, while the microbial community may still be changing at that time [16]. However, achieving this knowledge is difficult given typical MFCs experimental designs that allow anodic biofilm sampling only for limited time points during an experiment, which does not provide a direct linkage between the colonization process of microbial cells and power production. This is because sampling methods usually require sampling a considerable fraction of the biomass which necessitates mechanical disruption of the anode. We recently observed that efficient MFCs had _Desulforomodaceae_-dominated biofilms, while non-efficient MFCs showed a co-dominance of the anodic biofilm by both _Desulfuromonasace_ and _Geobacteraceae_[17]. These findings led to a better understanding of the important community dynamics that occur during the inoculation phase, and eventually lead to either an efficient or non-efficient MFC.