Highest growth rates do not correlate with resistance to heat
stress in UWO241.
To test whether growth temperature affects heat stress sensitivity,
UWO241 and C. reinhardtii were exposed to non-permissive
temperatures (24°C and 42°C, respectively). To ensure sufficient but
non-lethal stress, we based these treatments on previous heat stress
studies in UWO241 (Possmayer et al. 2011) and C.
reinhardtii (Hemme et al. 2014; Légeret et al. 2016),
which showed that the response of C. reinhardtii to 42°C was
broadly comparable to the that of UWO241 to 24°C. To quantify the
effects of heat stress, we measured cell death using Evans Blue dye that
accumulates in cells with damaged membranes (Supplemental Figure S1).
UWO241 cultured at 10°C and 15°C (temperatures that lead to the fastest
growth), completely lost viability after 72h exposure. In contrast,
UWO241 cultures grown at 4°C (with slower growth), were more resistant
to 24°C exposure and suffered only 57% cell death at 72h (Figure 4a).
Cell structures and chlorophyll were detected under the light microscope
only in the 4°C-grown UWO241 after 72h exposure to 24°C, but not in the
10°C and 15°C ones (Figure 4c). C. reinhardtii was most resistant
to 42°C when initially grown at of 28°C with fastest growth rates,
showing 30% cell death at 48h. Cultures acclimated to lower (10°C) and
higher (37°C) temperature were more sensitive to heat exposure and
rapidly lost viability (70% and 90%, respectively) by 48h. All
cultures appeared to be completely dead after 72h exposure (Figure 4b).
Similar patterns of chlorophyll loss (Supplementary Figure S2) and cell
structure were confirmed by light microscopy (Supplemental Figure S3).