2.3. Effect of seasonality and cultures’ depth on microalgal productivity in Fortaleza simulated-climate
In open raceway ponds, microalgae are exposed to a complex light environment. In addition to changes in sunlight intensity throughout the day and the seasons, as the culture reaches higher cell densities, light gradients occur within the water column and cells are exposed to different light intensities and short light cycles, depending on mixing efficiency [17]. This effect could be quite successfully simulated in our experimental system. Figure 4a shows a representative front view of the ePBR vessels for simulations ofS. obliquus C1S in 20-cm deep ponds at the inoculation time (OD750 = 0.3); at a later time (OD750 = 1.5); or simulations of 5-cm deep ponds at the same OD750 of 1.5, when all cultures were exposed to the same light intensity of 1,719 µmol photons · m-2 · s-1 at the surface of the water column. In Fortaleza simulated-climate, S. obliquus growth rate increased in simulated shallower ponds (Figure 4b), which resulted in a proportional increase in the volumetric biomass productivity of the cultures (Figure 4c). While volumetric productivity remained similar for summer and winter simulated-climate in 20- and 10-cm deep ponds, it resulted consistently higher in winter than in summer simulated-climate for 5-cm deep ponds (Figure 4c). As expected, a volume increase in deeper ponds compensated for the slower growth rate and similar areal productivities in both seasons in 20- or 10-cm deep ponds and in summer in 5-cm ponds were observed. However, increased growth rate in 5-cm ponds in winter resulted in a higher areal productivity of 26.22 ± 1.23 g · m-2 · d-1. The calculated annual mean productivities were 16.79 ± 1.92 g · m- 2 · d-1; 17.77 ± 0.97 g · m- 2 · d-1; and 23.01 ± 3.21 g · m- 2 · d-1, in 20-, 10-, or 5-cm ponds, respectively.
Interestingly, the low seasonality of biomass productivity determined for 20-cm ponds in Fortaleza, for a summer-to-winter ratio of 1.2, was further improved, and even reversed, in 10- and 5-cm ponds, for summer-to-winter ratios of 0.93 and 0.77, respectively.