Carbon constitutes half of the weight of the biomass, and it is usually supplied as CO2 (Gonzalez-Lopez et al. 2012). CO2 is normally added to the microalgae cultures as either air (generally no more than 1.0 % CO2) or flue gas (typically 10-20 % CO2) (Cheng et al. 2006). Environmental CO2 bioremediation using microalgae has a great potential, and many parameters have been investigated to optimize this process. Table 7.1 shows microalgae production parameters, including initial cell concentration, input CO2 concentration, aeration rates, temperature, light intensity, and PBR technologies, all normalized for CO2 fixation rates as g L-1 d-1. Using Tetraselmis sp. and Nannochloropsis oculata to fix CO2 from the air, it was reported an improved CO2 fixation rate for Tetraselmis sp. (0.0241 g L 1 d!) relative to N. oculata (0.017 g L-1 d-1) (Chik and Yahya 2012). Similarly, in another work, the CO2 sequestration rate for Dunaliella tertiolecta (0.12 g L-1 d-1) and Chlorella vulgaris (0.09 g L-1 d-1) was studied and it was demonstrated that D. tertiolecta fixed CO2 at a higher rate and greater efficiency than C. vulgaris (Hulatt and Thomas 2011). Furthermore, when 0.03 % CO2 was used for Chlorella pyr — enoidosa SJTU-2 and Scenedesmus obliquus SJTU, CO2 fixation rates of 0.134 and

0. 150 g L-1 d-1 were obtained, respectively, which showed higher fixation rates than those reported in the literature (Hulatt and Thomas 2011). The results pre­sented in Table 7.1 show that a wide range of CO2 biofixation rates are achieved (from 0.0241 to 0.150 g L 1 d!) from a number of different microalgae strains (Tetraselmis sp., N. oculata, C. vulgaris, D. tertiolecta, C. pyrenoidosa SJTU-2, S. obliquus SJTU, and Synechoccus sp.), using different PBRs (aerated flask, bubble column, and light diffusing optical fiber). The major differences in these publica­tions are the use of different algal species and the reported higher fixation rates that could be due to the use of two genetically engineered strains of microalgae (Tang et al. 2011). Determining the major parameters for increased CO2 fixation rates requires a more in depth study. For example, research on CO2 mitigation efficiency under 0.03, 0.55, and 1.10 % environmental CO2 concentrations for cyanobacte­rium Synechococcus sp. using light diffusing optical fibers (LDOF) showed 100, 33, and 15.4 % CO2 fixation efficiency, respectively (Takano et al. 1992). As the CO2 fixation efficiency decreased with increased CO2, the least percentage of input CO2 (0.03 %) attained the highest CO2 fixation efficiency (100 %), as supplying less CO2 allow microalgae cells to consume CO2 more efficiently. Yet, microalgal cell growth highly depends on supplied CO2 concentrations, and CO2 concentrations in air (0.03-1 %) do not commonly yield sufficient microalgal growth in PBRs. As industrial flue gases consist of higher CO2 concentrations suitable for microalgae production (Wang et al. 2008), using flue gas as feed for microalgae, not only can increase the productivity and cell growth of microalgae, but also remove CO2 from atmosphere more efficiently (Chen et al. 2012; Chiu et al. 2011). Furthermore, calcified microalgae (i. e., coccolithophorid microalgae) are of additional interests for CO2 bioremediation as they are able to form CaCO3 together with

photosynthetic carbon fixation (Moheimani et al. 2012b). In this case, carbon is removed by photosynthesis as a part of the carbon cycle, while the CaCO3 can be discharged (precipitated) out of the carbon cycle.