## 3 Microalgae Microbial Fuel Cell (mMFC)
### Microalgae and MFC  
In a conventional MFC, the oxygen is reduced at the cathodic surface consuming the electrons flowing from the anode through the external circuit. Because of that, oxygen availability used to be a limiting factor in the cathodic performance, mainly due to the cost of aeration and the diffusion dynamics [37; 74]. A cathodic microalgae culture, supplemented with CO\({}_{2}\) and nutrients, can be utilized to produce oxygen and other valuable compounds such as biomass and lipids. Moreover, employing biotic cathodes can enhance power outputs (from approximately 10 mW m\({}^{-2}\) to over 1 W m\({}^{-2}\)) [75] and lower aeration costs by replacing mechanical aeration systems with algal oxygen supply [76], which can also help reduce CO\({}_{2}\) emissions [21; 55; 77; 78]. Integrating photosynthesis with MFCs can be achieved through various configurations to facilitate the conversion of sunlight into electricity within the system [30; 74]. Among various algae strains, _Chlorella vulgaris_ has been extensively researched as a bioachode microorganism due to its ability to increase dissolved oxygen concentration and the oxygen reduction rate. This is attributed to its ease of cultivation, high growth rates, and lipid composition. The literature reports a maximum power generation of 38 mW m\({}^{-2}\) achieved with a carbon nanotube coating on the cathode, highlighting the importance of cathode coating for oxygen production [79]. Additionally, the algal biomass generated on the bioachode can be used as an endogenous carbon energy source for MFC anodes. This strategy could lead to a fully self-sustaining system [78; 80; 81] with high power densities of up to 1926 mW m\({}^{-2}\)[82]. Data presented in Table 2 and in Figure 4 indicate that _Chlorella_ spp. is the most frequently used microalgae species, accounting for 42.9% of the total, followed by microalgae consortia at 22.9%. _Spirulina_ spp. and _Scenedesmus_ spp. each representing 11.4%, with other species making up the remaining 11.4%. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Microalgae Spceic (Cathode)** & **MFC Configuration** & **Electrode** & **Maximum Power Density** & **Highlights** & **Reference** \\ \hline _Synchocystis_ + lagoon & Single chamber & Carbon cloths & 1.3 mW m\({}^{-2}\) & Positive response to light & [83] \\ microorganism & & & & & \\ \hline _Chlorella vulgaris_ & Double chamber & Glassy graphite rods & 2.7 mW m\({}^{-2}\) & Successful removal of CO\({}_{2}\) & [84] \\ \hline _Spirulina platensis_ & Single chamber & Platinum electrodes & 6.5 mW m\({}^{-2}\) & Higher power densities under non & [85] \\  & & & & & illuminated conditions & \\ \hline _Chlorella vulgaris_ & Double chamber & Carbon cloths & 13.5 mW m\({}^{-2}\) & Polarization resistance more & [86] \\  & & & & & significant at cathode & \\ \hline _Chlorella vulgaris_ & Double chamber & Plain graphite & 62.7 mW m\({}^{-2}\) & Increasing light intensity (96 \(\mu\)Em\({}^{-}\)s\({}^{-1}\)) enhanced power & [61] \\  & & & & & density by 600\% & \\ \hline Anaerobic mixed & Double chamber & Graphite plate & 57 mW m\({}^{-2}\) & Spring season yields 50 times & [87] \\  & & & & & greater power densities & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Configurations of mMFC systems. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Microalgae Specie** & & & & & \\
Figure 4: Quantification of studies. (**A**) Species used in the cathode. (**B**) mMFC configuration. (**C**) Anode materials. (**D**) Cathode materials. Different reactor configurations can be implemented, such as photosynthetic microalgae column [116], membrane algae reactors [89], photobioreactors (PBRs) [117], etc. These reactors can function as cathodic half-cells and be coupled with existing microbial processes such as yeast fermentation at bioethanol plants or wastewater treatment plants. Fermentative processes serve as anodic half-cells in creating coupled MFCs. Integrating MFCs into a bioethanol plant can generate some of the power needed for bioethanol production.