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
comparable physiologically to the that of UWO241 to 24°C.
To quantify the effects of heat stress, we measured the progressive loss
of chlorophyll and 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 their chlorophyll content and viability after 72h
exposure to 24°C. In contrast, UWO241 cultures grown at 4°C (with slower
growth), were more resistant to 24°C exposure and suffered only 57%
cell death and 72% chlorophyll loss at 72h (Figure 4A, C). Cell
structures and chlorophyll were visible 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 5).
C. reinhardtii was most resistant to 42°C when initially grown at
of 28°C with fastest growth rates, showing a 62% chlorophyll and a 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
chlorophyll (82% and 98%, respectively) and viability (70% and 90%,
respectively) by 48h. All cultures appeared to be completely dead after
72h exposure (Figure 4B, D). The loss of chlorophyll and cell structure
was confirmed by light microscopy (Supplemental Figure S2).
In all experiments, we returned the cultures to their original growth
temperature after 96h of heat exposure. All C. reinhardtiicultures recovered their growth, indicating that despite the apparent
full loss of viability in our measurements not all cells were dead even
after prolonged exposure to 42°C (Supplemental Figure 3A, B, C). In
contrast, UWO241 cultures did not recover once exposed to 24°C for 96h,
regardless of the initial culturing temperature, indicating a complete
loss of viability (Supplemental Figure 3D, E, F).