## 3 Microalgae Microbial Fuel Cell (mMFC)
### mMFC Configurations  
Figure 5 illustrates the four most common mMFC configurations [136; 137; 138]. In a coupled mMFC, a bioanodic MFC is connected to a PBR, where CO\({}_{2}\) is pumped directly from the MFC to the PBR. A single-chamber mMFC employs microorganisms capable of transferring electrons directly to the anode. In some cases, prokaryotic microalgae can act as electron donors at night and oxygen producers during the day. The most common mMFC design is the dual-chamber mMFC, which includes an anodic chamber with heterotrophic electrogenic bacteria and a cathodic chamber with microalgae, separated by a PEM. A photosynthetic sedimentary MFC (PSMFC) features an anode placed within a sediment layer covered with sand and a cathode compartment filled with a microalgal culture medium, leveraging natural electro-potential differences in a lagoon [37]. In single-chamber mMFCs, photosynthetic microbes such as _S. plantesis, Golenkinia_ sp. [139], mixed cultures [83], or _Chlorella_ sp. with mediators like methylene blue [140] can shuttle electrons to the electrode, serving as both electron donors and acceptors. This membrane-less configuration is cost-effective and features an aerobic cathode, and some microalgae species can adopt a mixotrophic metabolism, increasing the lipid content [37; 139]. Single-chamber configurations represent 8% of the studies in Table 2. Dual-chamber mMFCs are the most widely used configuration, accounting for 72% of the studies according to Table 2 and Figure 4. Generally, the dual-chamber configuration can be efficiently initiated through a three-stage process involving the separate production of bacteria and microalgae cultures, replacing the mechanical aeration system with a microalgae culture and finally adjusting the light dosage to promote photosynthesis [133]. Coupled mMFCs, which represent 12% of the configurations, according to Table 2 and Figure 4, use an MFC connected to a PBR, allowing for dual productive processes. An example includes ethanol production by yeast in the anodic chamber and microalgae biomass production in the cathodic chamber [81; 116]. In PSMFCs, oxygen mass transfer at the cathode is generally more efficient compared to other types of BESs. This efficiency is primarily due to the sufficient oxygen concentration produced by algae surrounding the cathode electrode. However, it is important to note that the high density of dissolved solids in sediments may still limit oxygen mass transfer in PSMFCs, potentially impacting their overall efficiency [83; 131].  
In the literature, a novel single-chamber mMFC configuration with a _C. vulgaris_ biofilm exposed to air was utilized to treat real dye textile wastewater [93]. A double-chamber mMFC with graphite plate electrodes operated for 32 days with _C. vulgaris_ and a bacterial community, showing promising results in both bioelectricity production and oil bioremediation [94]. Tubular polyethylene bags with graphite felt electrodes were used in a 10 L outdoor mMFC, demonstrating the feasibility of using low-cost materials to achieve high power densities (1200 mW m\({}^{-3}\)) with _Chlorella vulgaris_[98]. mMFCs can be applied as closed systems, for example, where microalgae are used as substrate in the anode and as live culture in the cathode [99]. Additionally, the anodic effluent of the mMFC can be utilized in the cathode chamber as a growth medium for microalgae with recirculation [101].