Bicarbonate (HCO3) is the predominant form of dissolved inorganic carbon (DIC) in seawater (pH = 8). At this pH, only 10 pM (less than 1 %) CO2 is present as DIC, leading to low CO2 diffusion for microalgal photosynthesis. The growth of mic­roalgae can lead to alkalization of the growth medium and occurs as a result of CO2 and HCO3- uptake and OH- efflux. With an insufficient supplementation of DIC, microalgal photosynthesis may decrease (Moheimani 2013). To assist higher photosynthetic productivity, most microalgae species have a carbon concentrating mechanism (CCM) allowing them to not only use CO2, but uptake HCO3- as a carbon source (Huertas et al. 2000). For instance, culturing two strains, Chlorella sp. and Tetraselmis suecica CS-187, in 120 L hanging bag PBRs by Moheimani is an example of using various inorganic carbon sources (industrial CO2, flue gas, and sodium bicarbonate) to grow algae. The highest biomass and lipid productivities of T. suecica (51.45 ± 2.67 mg biomass L-1 day-1 and 14.8 ± 2.46 mg lipid L-1 day-1) and Chlorella sp. (60.00 ± 2.4 mg biomass L 1 day 1 and 13.70 ± 1.35 mg lipid L 1 day 1) were achieved using CO2 as inorganic carbon source. When using pure CO2 or flue gases as a source of inorganic carbon, the specific growth rate, biomass, and lipid productivities of T. suecica were 23, 10, and 22 % higher than those with NaHCO3, respectively. Using pure CO2 or flue gases as a source of inorganic carbon, the biomass yield and both biomass and lipid productivities of Chlorella sp. were 6, 7, and 8 % higher than those with NaHCO3, respectively (Moheimani 2013). Growth rate of Thermosynechococcus sp. (TCL-1), under various DIC concentrations at a constant pH of 9.5 and temperature of 50 °C, increased with an increase in initial DIC concentrations (Fig. 7.2) (Hsueh et al.

2009) . The effects of DIC concentrations on TCL-1 growth were investigated at four DIC levels (Su et al. 2012). Using steady-state conditions and 28, 57, 85, and 113 mM DIC, the cell mass productivities of 486, 620, 948, and 1056 mg L-1 d-1 were achieved, respectively. The cell mass productivity was enhanced with increasing DIC, and thus, DIC is considered a limiting production factor. However, as DIC increased from 85 to 113 mM, the increase in cell mass productivity was only from 950 to 1050 mg L-1 d-1 (Fig. 7.3). Furthermore, the uptake of CO2 from bicarbonate by photosynthesis will release hydroxyl anion and increase the pH that

can be used as an indicator to confirm the alkalization process as a result of CO2 and/or HCO3- uptake. The inverse correlation between DIC and pH may be due to the buffer capacities of bicarbonate, as it is achieved under higher DIC levels and lead to smaller changes in pH (Su et al. 2012).