Each microalgae strain responds differently to CO2 concentration. Thus, the period of time that the CO2 concentration must be maintained in a medium culture depends on microalgal species. The two-stage growth technique (Suali and Sarbatly 2012) can be applied to increase the CO2 utilization of microalgae specifically during the

second stage by using up to 30% of CO2 and <10% of dissolved oxygen (DO) in media culture with a dispersion rate between 10 and 40% (Fabregas et al. 2001), whereas 20% of DO in air for aeration decreased the photosynthesis and lipid pro­ductivity of microalgae (Ota et al. 2011).

High CO2 concentrations can be utilized during the second-stage culture period to increase the lipid or oil production of microalgae. The productivity can be increased by adding as little as 2% of CO2 (Chiu et al. 2009). It is important to note that CO2 utilization by microalgae only occurs in the presence of light. Thus, excess CO2 that was not utilized during the photosynthesis could have an adverse effect on the productivity of the microalgae. By using light illumination as low as 25 pmol m-2 s-1, an aeration rate of 1 L min-1, and 5% CO2, biomass can be pro­duced that is 63% lipids (Illman et al. 2001). When cultivated in the range of 6-8% of CO2, the biomass yield can be as much as 0.376 g L-1 d-1, whereas 9-10% of CO2 resulted in a lower biomass yield of 0.15 g L-1 d-1 (Ghirardi et al. 2000; Metzger and Largeau 2005). A summary of first — and second-stage operating conditions is pro­vided in Table 14.2.