Initial cell concentration covers the range of 1-6.76 g L-1 which is summarized in Table 7.5. For example, two mutants of Chlorella sp. (MT-7 and MT-15) was investigated at different initial cell concentrations on CO2 fixation rates from 1 to 3 g L 1 (Ong et al. 2010). The CO2 fixation rate increased from 0.0124 to 0.0168 g L-1 d-1 and from 0.0109 to 0.0177 g L-1 d-1 for Chlorella sp. MT-7 and MT-15, respectively (Table 7.5), indicating a significantly higher CO2 fixation rate at higher biomass concentrations (Ong et al. 2010). Furthermore, Table 7.5 shows the effect of initial cell concentration on CO2 removal and biomass concentration of cyanobacterium Synechococcus sp. (Takano et al. 1992). When the initial micro­algal cell concentration increased from 1.4 to 6.8 g L-1, the CO2 fixation rate increased from 1.06 to 2.22 g L-1 and biomass concentration increased from 1.92 to 7.76 g L-1 (Takano et al. 1992). Additionally, CO2 retention times will be increased in higher biomass concentration as a result of higher culture medium viscosity, resulting in an enhanced CO2 removal rate and fixation efficiency (Ong et al. 2010).