3.3. Effect of seasonality and cultures’ depth on microalgal productivity in Fortaleza simulated-climate
Interestingly, no seasonality in biomass productivity was observed in the Northeastern region of the continent, especially in the more shallow simulated-ponds (10- and 5-cm deep). Conversely to most other sites analyzed in this study (Table 1 and Figure S2, Supporting information), and other worldwide reference sites for high microalgal productivity (Figure S4, Supporting information), in Fortaleza, and to a lesser extent in Caracas, increased raining and overcast days in the summer months result in less shiny days than in winter and spring. These results support the notion that, in addition to season-specific strains[14], season-specific optimization of culture conditions might have a positive impact on year-round productivity.
Results presented herein for climate-simulated shallow cultures ofS. obliquus fit very well with a previous outdoors cultivation study that used 2.6- to 4.4-cm deep open thin-layer cascade systems in Belgium in summer (irradiation of 1,500 – 1,700 µmol photons · m-2 · s-1 and 25 ± 5 °C) which showed a biomass productivity of 24 g · m-2 · d-1 [28]. Another study using a dynamic climate simulation for the cultivation of N. salina in thin-layer cascade systems under the summer climate of Almería, Spain, as a reference location for high productivity of outdoors cultivation of microalgae, also showed an areal biomass productivity of 25 g · m-2 · d-1[34]. Taken together, these studies indicated that, under favorable climate conditions, areal biomass productivities around 25 g · m-2 · d-1 are achievable in shallow ponds or thin-layer cascades either in true outdoors cultivation trials or under simulated-climate. This convergence of results is important for the validation of devices for the estimation of biomass productivity under conditions of simulated-climate, and also because productivity approaches the target towards commercially competitive production of microalgal biomass [15].
An evaporation rate of 5 - 7 L · m-2 · d-1 in shallow ponds and cascade systems operated under warm and dry conditions represents a challenging aspect of these cultivation systems [34]. Thus, even though these systems can be initially based on seawater, substantial amounts of freshwater would be needed to keep salinity within the microalgae cultivation range. This would be especially critical in Fortaleza because of its characteristic annual rainfall regime and the overlapping between the dry season and that of the highest biomass productivity potential. An elegant approach was proposed for collecting rainwater using the channel of thin-layer systems while storing the algae suspension in a tank [34]. Specifically dedicated resource-assessment and full techno-economic analyses are needed to evaluate whether long-term storage of rainwater would be feasible in Fortaleza.