Biomass Productivity
Table 1 summarizes the results obtained in different phases of temperature treatment. In Phase I, the average biomass productivity of cultures at constant 20 °C (in reactors AB & CD) and 35 °C (in reactors EF and GH) were 0.17 g L-1 day-1 and 0.21 g L-1 day-1, respectively. In Phase II, the cultures were shifted to contrasting temperature conditions. Average biomass productivities are given in Table 1 and show an apparent peak at 30 °C. In Phase III, the cultures were shifted to dynamic summer temperature profiles; average biomass productivities are shown in Table 1. The lowest biomass productivities were obtained in the ExSP (AB) treatment where the culture was shifted from 20 °C→35 °C→ExSP. Figure 3 summarizes the productivity results. The variation from 20 °C to 30 °C is less than the commonly observed Q10 = 2 behavior. This is due to the irradiance level (230 µE m-2s-1) being substantially below Pmax as shown below. The slight downturn at 35 °C and higher in Figure 3 is attributed to a stress response that is more apparent in the pigment results discussed below. The downturn in productivity and pigmentation was enhanced for batch experiments at 35 °C to 45 °C that are not discussed here.
The choice of 230 µE m-2 s-1 was based on the average annual irradiance at the exposed culture surfaces of PBR arrays in Fort Myers, Florida with a height-to-spacing ratio chosen to maximize productivity (Legere, 2017). To convert the quoted biomass productivities from g L-1d-1 to g m-2 d-1, multiply by a geometric factor of 95 L m-2. Thus, taking 30 °C as a reasonable estimate of the annual average daytime temperature in Fort Myers, a biomass productivity of 21 g m-2 day-1 is obtained. This is very close to the observed annual average of 23 g m-2day-1 found experimentally for large PBR arrays (24,000 L culture) tested for over 1 year at the Algenol site in Fort Myers (Chance and Roessler, 2019).