Various microalgae species accumulate biomass under different CO2 concentrations. Two experiments on S. obliquus supplied with 12 % CO2 resulted in 1.14 and 1.81 g L 1 as a maximum dry weight biomass yield, whereas a higher biomass concentration (3.5 g L-1) was obtained with Spirulina sp. when using a higher light intensity at the same 12 % CO2 input gas (Table 7.2) (De Morais and Costa 2007a, b). As shown in Table 7.2, the highest biomass concentration of Chlorococcum littorale in three runs under 20 % CO2 was 14.4 g L 1 in a small PBR (Kurano et al.

1995) . There has been several research publications evaluating the biomass pro­ductivity of different strains of microalgae under different CO2 concentrations (De Morais and Costa 2007a; Radmann et al. 2011; Yoo et al. 2010). Table 7.3 shows the biomass productivity of 0.077 g L-1 d-1 for Botryococcus braunii under

5.5 % CO2 (Yoo et al. 2010). A biomass productivity of 0.09 g L-1 d-1 for C. vulgaris was achieved (under 12 % CO2) (Radmann et al. 2011). Similarly, Spirulina sp. was used under 6 % CO2 and biomass productivity of 0.18 g L-1 d-1, signifi­cantly higher than most other reports, was obtained (De Morais and Costa 2007a). This higher biomass productivity might be due to lower CO2 concentration (6 %) used for Spirulina sp. in comparison with 12 and 20 % CO2 supplied for species in other reports. Similarly, microalgal species have shown differing capabilities for CO2 fixation at different concentrations of CO2 (10-20 % is a common range of CO2 for microalgae production systems with enhanced CO2 delivery (Ho et al. 2011)). The CO2 fixation rate of 0.17 g L-1 d-1 was obtained (Eberly and Ely 2012) for Thermosynechococcus elongates at 20 % CO2. Similar input CO2 concentration (20 %) (Tang et al. 2011) was supplied for S. obliquus SJTU-3 and C. pyrenoidosa SJTU-2, fixing 0.244 and 0.223 g L 1 d 1 CO2, respectively. However, the researchers also cultured the same strains under 10 % CO2 and obtained higher fixation rates, such as 0.288 g L-1 d-1 for S. obliquus SJTU-3 and 0.260 for C. pyrenoidosa SJTU-2, when providing high levels of CO2 into culture mediums that leads to acidification and lowering fixation rates of S. obliquus SJTU-3 and C. pyrenoidosa SJTU-2 at 20 % CO2. Research (Kurano et al. 1995) with C. littorale under 20 % CO2 assessed removing CO2 by 4, 0.65, and 0.85 g L-1 d-1 for selected culture volumes of 10 mL, 4 L and 20 L, respectively. The research indicated that C. littorale may achieve a better CO2 fixation rate than T. elongates, S. obliquus SJTU-3, and C. pyrenoidosa SJTU-2, although the differences were not significant. The higher CO2 fixation rate may be either due to engineering issues in scaling up or due to higher light intensity used for C. littorale (15650 lux) compared to other

investigations (11,200 lux) (Table 7.4). Light intensity controls photosynthetic growth in any microalgal system, CO2 removal rates, biomass concentrations, and overall growth rates. While increasing light intensity is usually accompanied by increasing CO2 removal rates in microalgal systems, any photosynthetic system has a saturation point where further increasing light intensity will either produce no benefit or may decrease productivity.