Biomass Productivity
Table 1 summarizes productivity results obtained in different phases of temperature treatment. These productivities are averaged over the entire phase and indicate an apparent peak at 30 °C. Figure 3 summarizes the productivity results found towards the end of the various phases with average values plotted against average daytime temperatures. This plot is intended to explore the final results of the extended periods at the various temperature conditions. The variation from 20 °C to 30 °C is attributed to the temperature dependence of Pmax (and thus Ek) and is less than the commonly observed Q10 = 2 behavior due to the irradiance level (230 µE m-2s-1) being substantially below Ek as shown below. This is the expected behavior when the system is stable and unstressed. No acclimation, other than the normal Ekdependence on temperature, is indicated. 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 consequences of the stress response are continuing throughout the time period of the phases, consistent with a degradation as opposed to an acclimation process. 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; see also Supplementary Material).