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].