CONCEPT AND RECENT DEVELOPMENTS

The urgent need for substantive net reductions in CO2 emissions into the atmosphere can be addressed via biological CO2 mitigation (Ramanan et al., 2009a, b; Fulke et al., 2010; Shekh et al., 2012; Yadav et al., 2012), coupled with a transition to value-added products (VAPs) such as biofuels (Fulke et al., 2010; Kumar et al., 2010). Microalgae can fix CO2 from the atmosphere, from flue gases, or directly as soluble carbonates by the process of photosynthesis using solar energy (Wang et al., 2008). Concurrently, biomass is produced with 10 to 15 times greater efficiency than terrestrial plants, which has application in carbon credit programs (Lam and Lee, 2011). Microalgal cells con­tain approximately 45% to 65% carbon, wherein 1 kg dry biomass is produced by fixing approximately 1.8 kg CO2 (Chisti, 2007). CO2 from the external atmosphere (air/ extracellular surroundings of microalgae) can be dissolved as bicarbonates and made available to microalgae for uptake and intracellular conversion to CO2 by intracellular carbonic anhydrases. CO2 is then made available to Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) for its fixation into energy compounds (Kaplan et al., 1991). Microalgae may provide a better tool for simultaneous CO2 sequestra­tion and biofuel generation. Current CO2 levels (0.0387% (v/v)) in the atmosphere are inefficient in supporting the high microalgal growth rates and biomass productivities needed for full-scale biofuel production (Kumar et al., 2010). Flue gases from various industries typically contain CO2 in the concentration range around 15% (v/v), which will provide sufficient amounts of CO2 for large-scale microalgae biomass production (Kumar et al., 2010). Owing to the cost of upstream separation of CO2 gas, direct utili­zation of power plant flue gas would be advantageous in microalgal biofuel production systems. Flue gases that contain CO2 concentrations ranging from 5% to 15% (v/v) have been scrubbed for direct use in microalgal culture systems for biomass growth (Kumar et al., 2010). This approach is believed to be pragmatic, more eco-friendly, and technologically feasible for bio-mitigation of CO2 as compared to physicochemical adsorbents or deep-ocean injections. This is a win-win scenario wherein combating air pollution through microalgal cultivation is possible while simultaneous microalgal biomass generation can be exploited to produce biofuel and other VAPs.

A comparative evaluation of CO2 sequestration potential of various microalgal species is presented in Table 11.1. Some microalgal species such as Chlorella, Scenedesmus, and Botryococcus are among the microalgae that have been studied for CO2 consump­tion and are promising for bio-mitigation of CO2 (Griffith and Harrison, 2009; Fulke et al, 2010). Scenedesmus obliquus was found to tolerate high CO2 concentrations (up to 12% v/v) with optimal removal efficiency of 67%, when grown at pilot scale using industrial flue gas as a carbon source (Li et al., 2011). Biomass generation through CO2 sequestration and exploitation of biomass for biodiesel precursor formation has been studied by Fulke et al. (2010). Chlorella sp. was found to have biomass productivity of 0.322 g L-1d-1 with lipid productivity of 0.161 g d-1 at 3% CO2 as feed gas.

The presence of FAMEs (fatty acid methyl esters) suitable for biodiesel (e. g., palmitic acid (C 16:0), docosapentaenoic acid (C 22:5), and docosahexaenoic acid (C 22:6)) have been confirmed. The calcite produced was characterized by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and x-ray dif­fraction (XRD) (Fulke et al., 2010). The ability to tolerate CO2 concentration during growth is confined to the individual specie’s characteristics. However, when exposed, the CO2 concentration in the gaseous phase does not provide a true reflection of the actual concentration of CO2 in the flue gas to which the microalgal specie is exposed during dynamic liquid suspension. It depends on the alkalinity (pH) and the CO2 concentration gradient created by the resistance to mass transfer (Kumar et al., 2010).