## 3 Microalgae Microbial Fuel Cell (mMFC)
### Electron Transfer Mechanisms  
The production of electricity in mMFCs depends on the redox potential of the electron acceptor availability [62]. The mechanisms of electron uptake for microalgae at the cathode are not yet fully understood [87]. Figure 7 presents four possible mechanisms for cathodic reactions in mMFCs [77; 84; 152]:  
* Direct CO2 reduction: Carbon dioxide is reduced without the involvement of microalgae or their metabolic products. * Direct electron transfer from cathode to algae: Algae directly receive electrons from the cathode, with CO2 acting as the electron acceptor. In this process, microorganisms may participate in reactions involving cytochrome C and indirect transfer mediators such as pyrroloquinolinequinone (PQQ) [153]. * Mediator-assisted electron transfer: This can occur with endogenous mediators produced by the microalgae or when using external mediators. In the literature, it has been proposed that when a mediator is utilized in the cathode compartment for electron transfer, electrons travel from the anode to the catholyte, where the oxidized mediator is reduced. The mediator then enters the microalgal cells, releasing its electrons, and returning to its oxidized state. The cultivated microalgae consume these transferred electrons in their metabolic pathways, enabling the conversion of CO2 into biomass and oxygen. The oxidized mediator is released back into the liquid bulk, beginning the cycle anew [37; 55; 84; 154]. For example, _C. vulgaris_ cultivated in a cathodic chamber acts as a final electron acceptor through electron transfer mediated by methylene blue and thionine blue [116]. In some cases, extracellular electron transfer may be supported by autonomous mediators, such as c-type cytochromes found in the outer membrane of algal cells [77]. * Oxygen reduction or indirect electron transfer: This occurs due to the reduction of oxygen produced during photosynthesis. Photosynthetic oxygenation of the cathode can happen in two ways: microalgae can be grown in separate photobioreactors, with oxygen transferred to the cathode chamber, or the algae can grow directly in the cathode chamber, generating oxygen in situ [154]. Figure 6: Power density comparisons. (**A**) Species used in the cathode. (**B**) mMFC configuration. (**C**) Electrode materials. The mMFC technology is based on the natural lagooning process that occurs when wastewater accumulates in ponds or basins [63]. In this process, a synergistic effect between heterotrophic microorganisms and algae takes place. Algae produce oxygen through photosynthesis, while heterotrophic microorganisms consume the organic matter in the water, utilizing the oxygen produced by the algae [37]. This integration allows algae to use solar energy and CO\({}_{2}\), making MFCs more environmentally and economically sustainable. The general biochemical reactions that occur during illumination are presented in Equation (1) for the anode and Equation (2) for the cathode [37; 67; 96]. \[\mathrm{C_{6}H_{12}O_{6}+6H_{2}O\to 6 CO_{2}+24H^{+}+24e^{-}} \tag{1}\]  
\[6\mathrm{CO_{2}+24H^{+}+24e^{-}\to CH_{1.71}O_{0.4}N_{0.15}P_{0.002}(microalgal biomass)+3O_{2}} \tag{2}\]  
During the light phase, algae perform photosynthesis producing oxygen and resulting in the highest electricity production in MFCs. In contrast, during the dark phase, when dissolved oxygen levels are at their lowest, electricity production suggests that nitrates or sulfates serve as electron acceptors [21]. Oxygen photosynthesis involves three membrane-bound protein complexes that sequentially transport electrons from water to NADP\({}^{+}\). Small, mobile molecules called plastoquinones and plastocyanins carry electrons between these large protein complexes over relatively long distances, playing a vital role in electron  
Figure 7: Possible mechanisms of cathode reaction in an mMFC. (**A**) Direct CO\({}_{2}\) reduction. (**B**) Direct electron transfer from cathode to algae. (**C**) Mediator-assisted electron transfer. (**D**) Oxygen reduction.