Since outdoor sunlight cannot be controlled, carbon fixation by microalgae is usually studied indoors under artificial illumination. A good deal of scientific effort is being made to evaluate microalgae CO2 fixation potential. Most of these efforts focus the fixation into biomass (Chae et al., 2006; Jacob-Lopes et al., 2008; Kajiwara et al., 1997). However, these studies did not quantify the total carbon dioxide fixed effectively by microalgae (Jacob-Lopes et al., 2008; Fan et al., 2007), since there are other routes for carbon besides biomass generation, such as mineralization (formation of soluble bicarbonate and carbonate) and production of extracellular products such as polysaccharides, volatile organic compounds (Shaw et al., 2003), organohalogens (Scarratt and Moore, 1996), hormones, and others.

The determination of global rates of carbon dioxide sequestration through mass balances of CO2 in the liquid or gas phase of the systems (Eriksen et al., 2007) gives more complete data. One approximation for the rates may be obtained by evaluating dissolved inorganic carbon concentration in the culture media while monitoring the pH variation (see methodology at Valdes et al., 2012). This shows that carbon fixation by microalgae is a complex process whereby biomass production might be a part of the total carbon destination. In addition, little information is available with respect to the simultaneous research of both the global rates of

02 consumed (g/h) —— 02 Base Line (g/h)

FIGURE 4.3 Gas phase analysis carried by Sydney et al. (2011) showing the carbon consumption and oxygen production profiles.

carbon dioxide sequestration and the rates of incorporation of carbon into the microalgae biomass (Chiu et al., 2008).

Sydney et al. (2011) studied the global CO2 fixation rate of four microalgae through a mass balance of the gas phase. The experiments were carried out in a photobioreactor coupled with sensors to measure CO2 in the inlet and outlet gases. The net carbon dioxide mitigation during each microalgal cultivation was evaluated. Nutrient consumption, biomass production (and composition), and possible extracellular products were analyzed throughout the process. It was found that between 70% and 88% of the carbon dioxide consumed was used in biomass production. This finding indicates that, to explore the whole potential of microalgal mitiga­tion capacity (considering negotiations in the carbon market), carbon balance might be carried through (complex) carbon balance in the gas phase. The problem is that it is difficult to carry out this kind of analysis in open photobioreactors and to standardize this methodology. Figure 4.3 presents the profile of carbon dioxide consumption obtained during gas phase analysis during cultivation. It is interesting to note that CO2 consumption (in blue) has a com­plementary behavior with O2 production due to photosynthesis and respiration processes during light and dark cycles.