8.4.1 CO2

CO2 fixation by microalgae through a photoautotrophic mechanism for harnessing liquid fuel is considered a reliable and sustainable approach for the neutralization of CO2 (Graham and Wilcox, 2000; Takagi et al., 2000; Ge et al., 2011; Wang et al., 2008; Yoo et al., 2010). Microalgae are considered as more photosynthetically efficient than terrestrial plants at fixing CO2 (Chiu et al., 2008; Indra et al., 2010). Microalgae also have the functional ability to fix CO2 from the atmosphere and industrial emissions (Brennan and Owende, 2010; Venkata Subhash et al., 2013). In the process of fixation, microalgae use CO2 as an inorganic carbon source, while water acts as an electron donor for the storage of reserve food material such as carbo­hydrates, which are further transformed to lipids under certain stress conditions. Many microalgae species are able to utilize carbonates such as Na2CO3 and NaHCO3 for cell growth (Wang et al., 2008). Most algae and cyanobacteria have different CO2-concentrating mecha­nisms (CCM) and act as enhancers for higher growth (Ramanan et al., 2010). CCM is activated only at low carbon levels and further depends on the strain, pH, light availability, and so on. The expression of the enzyme carbonic anhydrase (CA) has been associated with the induc­tion of the CCM. Chlorella sp., Spirulina sp., and Dunaliella sp. have been studied for CO2 se­questration. CO2 tolerance of Dunaliella sp. has been examined and used in the industrial production of beta-carotene (Graham and Wilcox, 2000). In Chlorella sp., growth was reported at 20% CO2 concentration (Hanagata et al., 1992). Scenedesmus obliquus and Spirulina showed good CO2 fixation rates when cultivated at 30 °C (Wang, et al., 2008). Mixotrophic cultivation of microalgae (mixed) by supplementing CO2 externally at different concentrations in domes­tic sewage showed enhanced biomass growth and lipid productivities (Prathima Devi and Venkata Mohan, 2012). The study documented functional advantages of the mixotrophic mode of nutrition. Photoautotrophic microalgae cultivation facilitates harnessing of renewable fuel in conjunction with CO2 fixation in a unified and sustainable approach. How­ever, algal cells cannot efficiently trap atmospheric CO2 to support the rapid growth needed for commercial operations (Duan and Savage, 2010).