## 1 Introduction  
The increasing world population has raised energy consumption substantially and motivated the search for cleaner and sustainable technologies [1]. In this context, several studies have attempted to improve bioelectrochemical systems (BESs), including microbial fuel cells (MFCs), to attain energy. Investigating new biocatalysts and modifying electrodes is fundamental to enhance microorganism-electrode interaction and to optimize extracellular electron transfer (EET), which are essential for further developing BESs [1].  
MFCs recognizably produce clean energy while simultaneously degrading organic compounds and pollutants [2]. Nowadays, this technology is mostly applied by using mixed cultures or microbial consorta. However, there is a gap in the knowledge about how some microorganisms, especially Gram-positive bacteria, contribute to generating energy in MFCs and their actual EET capacity [3].  
_Shewanella_ sp. and _Geobacter_ sp. have been extensively studied as pure cultures in MFCs [3,4]. Given that mixed cultures typically encompass numerous genera, the dominance of these species has prevented the electroactivity of other microorganisms from being explored. Furthermore, although Gram-positive bacteria are widely present in bioanodes, their electrogenic capacity has been poorly investigated. Compared to Gram-negative bacteria, Gram-positive bacteria have thicker cell walls, which may reduce their electron transfer efficiency [5]. This may be the reason why the EET of Gram-positive bacteria has been less frequently researched.  
Species belonging to the genus _Clostridium_ often occur in microbial communities of MFC bioanodes, and they act mainly as a fermenter [3; 6; 7]. However, the EET ability of _Clostridium_ sp. remains poorly explored. By using cyclic voltammetry, in 2014 [PERSON] et al. identified redox peaks for _C. pasteuranium_ DSM 525, which highlighted its significant electroactivity. The authors discussed the asymmetry of the reduction and oxidation peaks, which was comparable to the asymmetry observed in current-producing electroactive microorganisms [8].  
EET can occur through direct electron transfer (DET) or mediated electron transfer (MET) [9; 10]. Microorganisms that exhibit DET frequently contain redox proteins in their outer membrane, so they transfer electrons directly from these redox-active centers to the electrode. Nevertheless, some microorganisms that lack DET can carry out MET through redox mediator molecules [9]. For instance, _Pseudomonas aeruginosa_ and _Shewanella oneidensis_ are known for their ability to excrete phenazines and flavins, respectively, which can serve as electron mediators and enhance EET [11; 12].  
To improve EET, exogenous electron redox shuttles (mediators) can be added alongside microorganisms exhibiting low EET efficiency. The choice of mediator is critical, and criteria such as biological compatibility, stability, and redox potential should be considered [1; 13]. At non-toxic concentrations, soluble neutral red (NR), a phenazine-based molecule, and soluble methyl viologen (MV), a viologen-based molecule, can mediate electron transfer [1; 14]. Additionally, they provide fast electrochemical responses and efficiently operate at negative potentials, fulfilling the aforementioned criteria.  
Herein, we have assessed the ability of the Gram-positive _Clostridium pasteuranium_ strain DSM 525 to act as an electroactive biocatalyst in the anodic chamber of a glycerol-fed MFC, designated MFC\({}_{\text{DSM}}\). We will show that this bacterial strain can perform EET. We will also show that adding NR or MV in solution or as a film (FNR and FMV, respectively) to the MFC\({}_{\text{DSM}}\) anodic chamber, especially FNR, improves the MFC\({}_{\text{DSM}}\) performance and results in higher current densities.