## 3 Microalgae Microbial Fuel Cell (mMFC)
### Light Effect on mMFC  
Since light is the primary energy source for photosynthesis, it is crucial for the microalgae of the mMFCs to receive adequate light distribution to perform photosynthesis [96; 128]. Parameters associated with lighting, such as light irradiation, dark/light regime, and wavelength, have been studied in mMFCs [96]. This is because these factors influence chlorophyll formation, the photosynthesis process, and stomatal opening in microalgae cells [129]. Increasing light intensity from 26 to 96 \(\mu\)E m\({}^{-2}\) s\({}^{-1}\) in a biocathode with _Chlorella vulgaris_ results in approximately a six-fold increase in power production. The operation of the MFC with the _Chlorella vulgaris_ biocathode was enhanced using intermittent illumination. However, continuous illumination could reduce the lifespan of the microalgae [62]. Operating an mMFC during the spring, with a mixed microalgal culture, showed higher bioelectrogenic activity (57.0 mW m\({}^{-2}\)) compared to summer (1.1 mW m\({}^{-2}\)) when temperature and radiation were inhibitory for the algal culture. This demonstrated that in the microalgae biocathode, dissolved oxygen levels depend on the photosynthetic activity of the microalgae, which is related to factors such as temperature and light intensity [87]. Studies under different light irradiation conditions reveal that increased current outputs during light periods are associated with photosynthetic processes at the cathode. The in situ oxygen production by microalgae in mMFCs enhances the kinetics of cathodic reactions by reducing mass transport limitations for the ORR [130]. Rapid growth of microalgae can hinder light penetration in the photic zone of the cathodic chamber, halting growth [96]. Additionally, seasonal variations in light intensity impact oxygen production, affecting bioelectricity generation in MFCs [77]. The behavior of voltage peaks related to the dark/light phase and dissolved oxygen has been extensively explained in the literature [74]. Microalgal photosynthetic activity during light conditions increases the cathodic oxygen concentration, which increases the current exerted [90]. Conversely, under dark conditions, algae consume the available dissolved oxygen. As a result, the oxygen content at the cathode diminishes, leading to insufficient oxygen for reduction processes and a consequent drop in output current [131]. This dynamic of oxygen production and consumption causes peaks and valleys in the current generation trend [78]. The mMFC with _Chlorella vulgaris_ has shown power generation peaks during the day, with varying dissolved oxygen levels in the catholyte according to daily cycles. Dissolved oxygen (DO) levels were higher during the day (4.5 mg L\({}^{-1}\)) due to photosynthetic activity, while night-time levels decreased (1.2 mg L\({}^{-1}\)) because of respiratory oxygen consumption [132]. Chlorella vulgaris was studied in a double-chamber mMFC with plain graphite electrodes, finding that increasing light intensity (96 \(\mu\)E m\({}^{2}\) s\({}^{-1}\)) enhanced power density by 600% [61]. The impact of light intensity on mMFCs has been investigated for _Chlorella vulgaris_. An increase in continuous light intensity from 26 to 96 \(\mu\)E m\({}^{2}\) s\({}^{-1}\) resulted in approximately a six-fold increase in power production. This enhancement is likely due to higher light intensity promoting greater photosynthetic activity and oxygen production. Additionally, the concentration of microalgal biomass was positively affected by high light intensity, leading to a growth rate up to ten times higher than that achieved under lower light intensity [61]. The results indicate that the increased current outputs during photoperiods are associated with photosynthetic processes. The in situ bio-oxygen produced reduces mass transport limitations for the ORR, thereby improving the kinetics of the cathodic semi-reaction, which is typically a rate-limiting step in MFCs [130].