Photoprotection is regulated by light-independent CO2 availability

Photosynthetic algae cope with suboptimal levels of light and CO2. In low CO2 and excess light, the green alga Chlamydomonas reinhardtii activates a CO2 Concentrating Mechanism (CCM) and photoprotection; the latter is mediated by LHCSR1/3 and PSBS. How light and CO2 signals converge to regulate photoprotective responses remains unclear. Here we show that excess light activates expression of photoprotective and CCM-related genes and that depletion of CO2 drives these responses, even in total darkness. High CO2 levels, derived from respiration or impaired photosynthetic fixation, repress LHCSR3 and CCM genes while stabilizing the LHCSR1 protein. We also show that CIA5, which controls CCM genes, is a major regulator of photoprotection, elevating LHCSR3 and PSBS transcript accumulation while inhibiting LHCSR1 accumulation. Our work emphasizes the importance of CO2 in regulating photoprotection and the CCM, demonstrating that the impact of light on photoprotection is often indirect and reflects intracellular CO2 levels.