## 2 Bioelectrochemical Systems' Configurations  
BESs, also referred to as Microbial Electrochemical Technology (MET), are composed of two electrodes, an anode and a cathode, connected by an external electric circuit. In the anode, exoelectrogenic microorganisms act as biocatalysts, transferring electrons to the electrode through the anaerobic oxidation of organic substrates. Meanwhile, the cathodic chamber acts as a sink for the electrons received from the anode. When both anodic and cathodic reactions occur spontaneously, the system is referred to as MFC. However, when the reactions are not spontaneous, and require energy consumption, the devices are named microbial electrolysis cells (MECs). Table 1 describes several characteristics of the different types of BESs, such as configurations, anodic and cathodic materials, type of microorganisms used, substrates, electron acceptors, among others. These BESs are also classified according to their configuration and application as shown in Figure 3:  
* Microbial Fuel Cells (MFCs) can generate electricity by performing oxidation and reduction reactions; usually, the reduction reaction is based on the oxygen reduction to water due to its high redox potential [34; 35]. A typical MFC consists of an anodic and a cathodic chamber, separated by a proton exchange membrane (PEM). In the anodic chamber, microorganisms metabolize organic substrates to produce electrons and protons. The redox potential difference between the electron donor, usually an organic substrate at the anode, and the electron acceptor, typically oxygen at the cathode, causes electrons to move to the surface of the anode and then flow to the cathode through an external electrical circuit. Meanwhile, protons migrate to the cathode through the PEM [36]. In the cathode chamber, electrons and protons combine to reduce oxygen to water [37]. MFCs are among the most studied types of BES. For instance, a single-chamber MFC with a carbon brush anode and a platinum (Pt) cathode was used with a 1:1 ratio of wood to domestic wastewater [38]. The results were significant, showing an 85% removal of Chemical Oxygen Demand (COD), an 18% Coulombic Efficiency (CE), and a power density (PD) of 360 mW m\({}^{-2}\) with a current density (CD) of 1.3 A m\({}^{-2}\). Additionally, this setup allowed for a comprehensive analysis of biological diversity within the system [38]. Double-chamber MFCs utilized rice husk charcoal electrodes in wastewater from poultry droppings. This configuration achieved a PD of 287 mW m\({}^{-2}\) and a CD of 0.51 A m\({}^{-2}\), with a 40% reduction in COD. The use of new materials and real wastewater further validated the practical applications of this setup [39]. * Microalgal Microbial Fuel Cells (mMFCs) could be described as a type of MFC that uses microalgae as a biocatalyst in the anodic or cathodic chamber. These devices will be discussed in detail in the next sections. * Microbial Electrosynthesis Cells (MES) can reduce chemical compounds to generate different value-added chemicals. In this configuration, the anodic oxidation reaction is similar to the MFC's; however, the cathodic reaction in a MEC requires additional energy to drive the non-spontaneous overall reaction to obtain the desired products. Several studies on MES systems demonstrated the versatility of this technology. For example, a two-chamber MES with polished graphite plate electrodes and 35 mM acetate at \(-0.8\) V showed that sulfate-reducing bacteria could enhance electron transfer within the cathode biofilm, leading to a symbiotic effect among the bacteria [40]. A two-chambered planar carbon felt electrode setup with 2.7 mM acetic acid at \(-0.8\) V led to higher specialization within the biofilm, which improved acetate generation and demonstrated an important symbiotic relationship with bacteria [41]. * Microbial electrolysis cells (MECs) can produce hydrogen gas and/or enrich metabolites through the electrolysis of water with externally applied voltage [15].