Moderate High Temperature is Beneficial or Detrimental Depending on Carbon Availability in the Green Alga Chlamydomonas reinhardtii

High temperatures impair plant/algal growth and reduce food/biofuel production, but the underlying mechanisms remain elusive. The unicellular green alga Chlamydomonas reinhardtii is a superior model to study heat responses in photosynthetic cells due to its fast growth rate, many similarities in cellular processes to land plants, simple and sequenced genome, and ample genetic, genomic, and high-throughput tools. The interaction of heat responses with the availability of organic carbon sources is important for the algal biofuel/bioproduct industry but understudied. We cultivated Chlamydomonas wild-type cultures under highly controlled conditions in photobioreactors at control of 25°C, moderate high temperature of 35°C, or acute high temperature of 40°C with and without constant organic carbon supply (acetate) for 1- or 4-days. Our results showed that moderate high temperature was beneficial to algal growth with constant acetate supply but detrimental without it. The overlooked and dynamic effects of 35°C can be explained by induced carbon metabolisms, including acetate uptake/assimilation, glyoxylate cycle, gluconeogenesis pathways, and glycolysis. Acute high temperature at 40°C for more than 2 days was lethal to algal cultures even with constant acetate supply. Our research provides insights to understand algal heat responses and help improve thermotolerance in photosynthetic cells. Highlight We revealed the overlooked, dynamic effects of moderate high temperature in algae with different carbon availabilities and demonstrated the importance of carbon metabolism in thermotolerance of photosynthetic cells.