## 3 Microalgae Microbial Fuel Cell (mMFC)  
### Microalgae  
Microalgae are small, single-celled, photosynthetic organisms that can be either prokaryotic or eukaryotic. The microalgae are capable of producing a broad range of commercial compounds, such as proteins, \(\omega\)-3 fatty acids, pigments, cosmetics, different value-added chemical products, as well as third-generation biofuels [60]. The growth of microalgae is influenced by various factors, including light, temperature, nutrient availability, and pH levels [61, 62, 63].  
Microalgae cultures possess several interesting characteristics that make them highly applicable in biotechnological processes. The ability to grow in different kinds of waters, removing pollutants from wastewater, consuming CO\({}_{2}\) as a carbon source and thus mitigating GHG emissions (6.24 kg of microalgae biomass per m\({}^{3}\) CO\({}_{2}\)), their ability to generate value-added products [64, 65], their tolerance to a wide variety of environmental conditions, the high biomass productivity per unit area, high photosynthetic efficiency and fast growth, among others, make the microalgae very interesting biocatalysts mainly when dealing with wastewaters [66, 67].  
Algae-based CO\({}_{2}\) capture surpasses terrestrial sequestration by one to two orders of magnitude. Algae play a crucial role in generating about 75% of the atmosphere's oxygen, only depending on the access to CO\({}_{2}\) and light [61]. The use of microalgae for biofuel production represents one of the most direct applications, given their high lipid content (over 30% by weight) and their capacity to CO\({}_{2}\) capture [65].  
One of the most crucial factors influencing the growth and makeup of microalgae biomass (fatty acid and pigment profiles) is light (quality, intensity, and dark/light regimes). Light intensity influences photosynthesis, CO\({}_{2}\) removal rates, biomass concentration, and overall growth rates [68]. Generally, increasing light intensity boosts the growth rate of microalgae [69]. However, excessively high light intensities can lead to photo-inhibition. Conversely, low light intensities may restrict growth and increase chlorophyll content. It is well documented that cultivation under stress conditions, such as very high or low light intensities, can alter the composition to favor lipid accumulation, depending on the algal species and its ability to capture CO\({}_{2}\) under high light conditions [70].  
Nutrients play a critical role in the growth and composition of biomass [61; 62]. The lipid production in microalgae is determined by the species' inherent capacity (genotype/phenotype) and abiotic factors such as wavelength, light intensity, and the composition of the culture medium (primarily carbon source, nitrogen, and phosphorus) [71]. Carbon is the primary component of biomass, comprising 50% by weight. The concentration of CO\({}_{2}\) in the air is 380 ppm, and the dry weight composition of algae requires 1.8 kg of CO\({}_{2}\) per kg of biomass. For microalgae culture, additional carbon is supplied through aeration, enriched air, or organic sources during photosynthesis to form various organic metabolites [71; 72; 73]. Optimal CO\({}_{2}\) concentration values vary not only among different microalgae strains but also for the same strain under slightly different conditions.