Outdoor Simulation: Phase III
Phase III involved exposing the cultures to dynamic temperature profiles with hourly changes in temperature that are representative of outdoor summer culture temperature profiles (Supplementary Material). This final phase of cultivation was carried out from day 33-43 with scheduled alternate day dilutions and with no adaptation period. The cultures grown at constant 20 °C (EF) and one set of the constant 30 °C (CD) cultures were shifted to average summer profile (AvSP) with 35 °C/21 °C as maximum/minimum temperatures during the course of day/night temperature ramping. The cultures at constant 35 °C (AB) were shifted to an extreme summer profile (ExSP) with 39 °C/26 °C as maximum/minimum temperatures, and the final set of two photobioreactors at constant 30 °C (GH) were shifted to constant summer profile (CtSP) where day/night temperature were maintained at constant 31 °C/21 °C, selected based on Fort Myers summer profile and averaging the day and night temperatures separately.
The average pre-dilution cell concentration in gDW L-1for the temperature profile treatments are shown in Table 1. The relative dilution rates were about 0.16 day-1 in AvSP (CD), AvSP (EF) and CtSP (GH), and about 20% lower (0.135 day-1) for ExSP (AB).
From visual observations, it is worth noting that during the processing of biomass samples for the various analyses, agglomeration or clumping of the algal cultures occurred for cultures grown at higher temperature (constant 35 °C and ExSP). This is attributed to a stress response. In addition, the dried samples from these temperature exposures showed a flaky texture on the dry weight plate membrane surface. In cyanobacteria, high temperature stress can result in a rise in fluidity of membranes which can cause disintegration of the lipid bilayer and many other alterations in the physical properties of the cells that result in the loss of functionality of photosynthetic machinery (Panyakampol et al., 2015; Panyakampol et al., 2016).