Fig. 1. Moderate (35°C) and acute (40°C) high temperatures had contrasting effects on the growth rates of Chlamydomonas cells.

a Chlamydomonas growth rate plateaued around 35°C but was largely reduced at 40°C. Chlamydomonas cells (CC-1690, 21gr, wildtype) were grown in photobioreactors (PBRs) in Tris-acetate-phosphate (TAP) medium under turbidostatic conditions at different temperatures with a light intensity of 100 µmol photons m⁻² s⁻¹ and constantly bubbling of air. Relative growth rates were calculated based on the cycling of OD₆₈₀, see Supplementary Fig. 1 and methods for details. OD₆₈₀ is proportional to total chlorophyll content in unit of µg chlorophyll mL⁻¹. Each temperature treatment was conducted in an individual PBR. Mean ± SE, n = 3 biological replicates. Statistical analyses were performed using two-tailed t-test assuming unequal variance by comparing treated samples with pre-heat or 30°C with 35°C (*, p < 0.05, the colors of asterisks match the treatment conditions). Not significant, ns. b Heat treatment at 35°C (brown) increased growth rates while 40°C (red) reduced it. Algal cultures in separate PBRs were first acclimated at 25°C for 4 days before the temperature was switched to 35°C or 40°C for 24 h, followed by recovery at 25°C for 48 h. Algal cultures grown constantly at 25°C (black) served as controls, which demonstrated steady growth without heat treatments. Three independent biological replicates for each condition were plotted. c PBR cultures with different treatments were sampled at a series of time points to study heat responses at multiple levels. Symbol colors match the colors of the parameters assayed. b, c The red shaded areas depict the duration of high temperature.