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.