3.2. Effect of CO2 supplementation and seasonality on microalgal productivity simulated-climate of different regions
Atmospheric concentration of CO2 and its diffusion into water normally limit the maximum phototrophic growth rate and biomass productivity of S. obliquus [28] and most microalgae [29]. It was estimated that to achieve a biomass productivity of 25 g ⋅ m-2 ⋅ d-1, a minimum of 46 g ⋅ m-2 ⋅ d-1 of CO2 must be dissolved in the culture medium to avoid its limitation on biomass productivity[30]. Although the cost of raw materials represents a low fraction of algal biomass production cost in comparison with facilities, machinery and labor, CO2 is currently the main contributory factor to utility costs[10,29]. The utilization of flue gas from industrial waste as a source of CO2 is likely the best available alternative [29]. However, the logistics related to availability, management and transportation imposes additional costs. Resource assessment analysis suggested that access to flue gas by transportation from emission sources located 4.8 km or 80 km from the microalgae farm in the US would render 44 or 1,800 million barrels of oil equivalent, respectively. Being the latter option clearly not economically competitive [29]. A comparatively lower value-added industrialization in South America[31,32], is correlated with a lower production of CO2 as an industrial waste [33].
Lack of CO2 supplementation lessened annual mean biomass productivity more than 50% mostly regardless of the geographical site. A similar study aimed at studying the influence of CO2supplementation on S. obliquus outdoors cultivation in 2.6- to 4.4-cm deep open thin-layer cascade systems in Belgium in summer showed that biomass productivity dropped to 46% of that of the CO2-supplemented cultures (56% lower)[28]. Taken together, regardless of differences in the culture systems, these studies tend to indicate the need for CO2 supplementation to boost biomass productivity about two-fold, which otherwise would leave biomass productivity quite far away from productivity targets for commercial feasibility[15].