Biodiesel is mainly derived from the intracellular lipids of oleaginous microalgae. Algal lipids consist of triglycerides (TAG) along with mono and diglycerides and free fatty acids. Stearic acid, palmitic acid, and oleic acid are the predominant fatty acids types in the algal lipids (Tripathi et al., 2015). Algae biomass displays a variable amount of lipid content depending on the strain type and cultivation condition. For example, the lipid content of Chlorella vulgaris varies from 11% to 43% (Mitra et al., 2012). Enamala et al. (2018) reviewed several studies and found that the lipid content varies from 2.4% to 62% of dry algal cell weight (Enamala et al., 2018).

Algal lipids are converted to biodiesel through a catalytic transesterification reaction between triacylglycerols and methanol. The transesterification process results in fatty acid methyl ester (FAME) and glycerol (Kumar et al., 2020). The purified FAMEs are known as biodiesel. The lipid composition, such as the percentage of saturated fatty acids, affects the fuel properties. The algal oil contains more unsaturated fatty acids than saturated ones, which improves cold flow and make it a suitable feedstock (Demirbaş, 2009; Tripathi et al., 2015). However, it also triggers the production of hydroperoxide and insoluble substances which collectively lead to choking of the filter (Kumar and Thakur, 2018).

Algae biomass or leftover algal biomass after lipid extraction (Lipid extracted algae) produces biogas when subjected to anaerobic digestion. This biogas is composed of CH₄ (50-70%) and CO₂ (30-50%) (Kumar et al., 2020). The algal biomass can generate 0.024 –0.6 L CH₄/g VS (volatile solid) or 0.2 –0.4 m³ CH₄/kg biomass. The biogas yields vary from one species to another and depend on process conditions (Milledge et al., 2019; Rabii et al., 2019). The factors affecting biogas production include algae cell wall composition, process temperature, C/N ratio, biomass loading rate and reactor configuration (Sialve et al., 2009; McKennedy and Sherlock, 2015; Barbot et al., 2016). The biogas production process when integrated with other bioenergy processes adds value and makes it sustainable (Cesaro and Belgiorno, 2015).

Biochar is produced through hydrothermal carbonization (HTC) of dry biomass (Gollakota et al., 2018). Algae based biochar has a high cation exchange capacity, lesser carbon proportion, and lesser surface area than lignocellulosic biomass based biochar (Michalak et al., 2019). Algae based biochar has higher yield compared to other feedstocks and the yield ranges from 8.1% to 64.2% of dry biomass (Yu et al., 2017; Michalak et al., 2019). The high ash content blocks micropores resulting in a low active surface area (Leng et al., 2021). Algae biochar also possesses several functional groups making it suitable for the remediation of inorganic and organic contaminants from wastewater (Kumar et al., 2020).

Algal biomass can ferment to bioethanol under anaerobic conditions. The process is mediated by yeast, bacteria and/or fungi (Minh and Hanh, 2012; Robak and Balcerek, 2018). The algae biomass contains several polymers like mannitol and agar (Kostas et al., 2016; Offei et al., 2018). Brown algae are rich in carbohydrates such as alginate, mannitol, laminarin, glucose, fucoidan, and cellulose (Ale and Meyer, 2013). Similarly, red algae have a diverse range of hydrolysable polymers, which can be converted to ethanol (Behera et al., 2015).

Intact algae biomass or algae products find applications in industries such as food, pharmaceutical, healthcare and cosmetics. Algal species such as Spirulina and Chlorella serve as a food supplement and source of protein (Kumar et al., 2020). Algae produce pigments like carotenoids, phycocyanin and chlorophyll (Barkia et al., 2019). Carotenoids such as zeaxanthin, α-carotene, β-carotene, and lutein are antioxidants and have anticancer properties (Dickinson et al., 2017; Matos, 2017). The polyunsaturated fatty acids (PUFA) derived from algae serve as food supplements (Lee, 2013). In addition to this, some unconventional value-added products such as ubiquinone coenzyme Q₁₀, ubiquinol, cannabinoids, anandamids, hoshinolactum, dolastatins, endotoxins and several therapeutic substances can be obtained from algae (Abu-ghosh et al., 2021; Hans et al., 2021; Mondal et al., 2020).

The electrigens reduce the anode by oxidizing the organic matter present in the anodic chamber. The process of anode reduction is thermodynamically favorable and hence spontaneous. The anodic redox potential is dependent upon the chemical nature of organic matter and can be calculated using the well-known Nernst equation. On the other hand, electrons in the cathodic chamber are commonly accepted by oxygen due to their availability and high redox potential (+0.82 V). Still, a number of other chemical acceptors are also used that include nitrate, manganese oxide, iron, hydrogen peroxide and nitrite (Chaudhuri and Lovley, 2003). The schematic representation of MFC is shown in Figure 2 . The basic functional mechanisms of MFC would be clearer by an example of reactions on the electrodes surface, as shown below:

Anode:

Cathode:

The overall cell voltage can be described as:

The ideal cell voltage that a system can generate is represented by equation 2.3, but due to the association of several losses in real MFC, the operating voltage is lowered. Primarily, there are 3 integral components of a typical MFC, namely anode, cathode and proton/cation exchange membrane.

As mentioned earlier, algae assisted MFCs hold significant promise in making MFC technology sustainable. Algae-assisted MFCs can be powered by low-cost algae biomass; can produce algae biomass which serves the dual purpose of carbon capture, and oxygen generation. Oxygen is the most preferred electron acceptor in MFCs as it supports high potential differences. Algae cultivation at the cathode provides the system with a continuous supply of oxygen (during the light period) and helps circumvent the installation of mechanical aerators. Algae biomass also serves as feedstock for biodiesel generation and several other products. Wang et al. reported a power density of 5.6 W/m³ in a Chlorella based MFC (Wang et al., 2010). A culture of cyanobacteria, Anabaena, at the cathode sparged with a CO₂-air mixture gave a power density of 57.8 mW/m² (Pandit et al., 2012). In a study, a power density of 2.48 W/m³ and a Coulombic efficiency (CE) of 9.4% were attained using immobilized algae systems (Zhou et al., 2012). Photosynthetic microbial fuel cell (PMFC), algae assisted microbial fuel cell (AMFC) or microbial carbon capture cell (MCCs) also serve as a modified photo bioreactor equipped with an inherent oxygen quenching mechanism and carbon dioxide supply. The process of algae cultivation at the cathode also complements the effective carbon removal at the anode. Microalgae biomass is rich in hydrolysable carbohydrates, fats, and proteins and can serve as an electron donor substrate at the anode (Cui et al., 2014).

An algae assisted MFC can take different configurations depending on the intended application, i.e., algal production, power generation and wastewater treatment. Various kinds of algae-based MFC configurations are shown in Figure 3 . The configuration varies from triple chamber to single chamber. A single chamber algae assisted MFC involves bacterial and algae cultivation in the same chamber. The CO₂ produced by bacteria is effectively sequestered by the microalgae present in the same chamber. The carbon capture is efficient and the system is easily maintainable. In a two chambered system, algae and bacterial consortia are separated by a proton exchange membrane (PEM). These systems are used for algae cultivation for bioenergy or other applications. A separate photo-bioreactor is sometimes coupled with the system to enhance the algae growth rate and power generation. A three chamber algae-based MFC finds application in water desalination, where saltwater is fed to the middle chamber to facilitate flow of positive and negative ions. Researchers have also used uplift aeration type MFC to support high algae growth rates (Saba et al., 2017).

Algae biomass is rich in decomposable carbohydrates, lipids and proteins. Therefore, algae serve as a good source of electron donor at the anode. The primary challenge with the use of intact algae biomass is its complex cell wall. Algae cell wall composition varies from class to class and species to species. The chlorophycophyta contain a wide array of cell walls ranging from cellulose pectin complexes to hydroxyproline rich glycoproteins. Like plants cells, algae cell walls are intricate mix of polymers such as cellulose, hemicellulose, lignin, pectin and arabinogalactan proteins. This complex assembly of polymers in the algae cell wall necessitates the biomass pretreatment to break open the structure, enhance the surface area, and hydrolyze polymers. Researchers have used both intact and pretreated micro and macroalgae as anodic substrates and reported good power outputs (Velasquez-orta et al., 2009; Cui et al., 2014). The use of pre-digested algae biomass also supports high power output over undigested biomass (Salar-garcía et al., 2016).

Algae at the cathode not only serve as oxygen supplier but also as catalysts for oxygen reduction at the electrode surface. Algae produced metabolites also serve as electron acceptors in the absence of oxygen particularly during the dark period. The success of algae assisted MFC depends on the process of photosynthesis which is driven by light energy and carbon dioxide supply (González Del Campo et al., 2013; Elmekawy et al., 2014).

Additionally, algae can effectively remove nitrates and phosphates from the water. Algae can grow in autotrophic, heterotrophic, and mixotrophic mode. Heterotrophic and mixotrophic modes assist with carbon removal. The simultaneous carbon, nitrogen, and phosphorus removal is possible using dual chamber algae assisted MFCs. The anode and cathode both can contribute towards carbon removal while the algae assisted cathode can help with nitrogen and phosphorus removal (Commault et al., 2017). The success of algae assisted MFC in wastewater treatment depends on the algal strain, inoculum size, density, temperature, N/P ratio, salinity, pH, light intensity and CO₂ supply and capture rate. An algae assisted MFC thus needs optimization with respect to all these parameters (Nagendranatha Reddy et al. 2019).

Light is the primary requirement for photosynthesis. Light intensity, its duration (light/dark period), and wavelength all affect algae growth. High light intensities lead to photo oxidation and growth inhibition. On the other hand, low light intensities lower algae growth rates and promote bacterial growth. Both polychromatic and monochromatic light is used for cultivating algae. Amongst the monochromatic light, the red and blue light is most preferred for high rate algal cultures. Light source and its orientation with respect to MFC affect algae growth. Researchers prefer using inbuilt LED lights that ensure direct illumination and minimize the self-shading effect. It also helps to regulate temperature and ensure low temperatures enabling optimized algae growth. The ratio of light/dark period is also critical and varies from species to species and system to system (Saba et al., 2017; Nagendranatha Reddy et al. 2019).

Carbon dioxide is another key ingredient required for algae growth. Most of the algae grow well at atmospheric CO₂ levels. However, higher concentrations are shown to promote algae growth and carbon capture (Singh and Singh, 2014). Researchers have studied the impact CO₂ concentration on growth and lipid production in algae (Sato et al., 2003; Wang et al., 2010). The response varies from species to species and also on the cultivation conditions. In an algae assisted MFC, the CO₂ required by the microalgae can either be the CO₂ present in the anodic off-gas (Wang et al., 2010) or it can be CO₂ sparged separately (González Del Campo et al., 2013). However, CO₂ sparging is associated with certain disadvantages, including the lowering of the pH on the dissolution of CO₂ in water, which can be resolved by the use of a higher initial inoculum concentration (Chiu et al., 2009; Zhang et al., 2014). Increasing the CO₂ concentration by 10-15% has resulted in a 6% increase in the lipid content, confirming the significance of the CO₂ concentration in the lipid content of algal cells (Liu et al., 2011).

Photosynthesis liberates oxygen via light reaction and algae consume oxygen while respiring. A high concentration of oxygen becomes inhibitory for algae growth and leads to photo-oxidative damage. It was found that a DO concentration exceeding 30 mg/l inhibited the C. vulgaris growth by 30% (Kazbar et al., 2019). MFC circumvents this problem as oxygen is quenched through the reduction reaction in a circuit MFC. The solubility of oxygen in water is also dependent on temperature, salt content, and duration of light/dark cycles. It is often observed that during night time, the DO level drops and so is the power output from a MFC (Gonzalez Del Campo et al., 2015). A DO level of 4.5-5.5 mg/l is suitable for supporting continuous power output from a MFC (Rodrigo et al., 2010), while an algae based cathode can realize DO levels in the order of 6.6 mg/l (Kang et al., 2003). Another major factor that determines the DO concentration in water is temperature. Hence, the use of proper lighting equipment with useful wavelengths is of utmost importance. In other words, DO is dependent on the temperature as well as the duration of the light/dark cycles. One important investigation to show this relationship was the one carried out by Gouveia et al. (2014) wherein experiments were conducted using two light intensities 26 µE/m² and 96 µE/m². An increase in algal growth rate with increased oxygen concentration resulting in enhanced power generation at higher light intensity was observed in this investigation (Gouveia et al., 2014).