Joint operation of the CO ₂ concentrating mechanism and photorespiration in green algae during acclimation to limiting CO ₂

Due to low availability of CO ₂ in aquatic environment, microalgae have evolved a CO ₂ concentrating mechanism (CCM). It has long been thought that operation of CCM would suppress photorespiration by increasing the CO ₂ concentration at the Rubisco active site, but experimental evidence is scarce. To better explore the function of photorespiration in algae, we first characterized a Chlamydomonas reinhardtii mutant defected in low-CO ₂ inducible 20 (LCI20) and show that LCI20 is a chloroplast-envelope glutamate/malate transporter playing a role in photorespiration. By monitoring growth and glycolate excretion in mutants deficient in either CCM or photorespiration, we conclude that: i . CCM induction does not depend on photorespiration, ii . glycolate excretion protects algal cells from the toxicity of unmetabolized photorespiratory intermediates, iii . photorespiration is active at low CO ₂ when the CCM is operational. This work provides a foundation for a better understanding of the carbon cycle in the ocean where significant glycolate concentrations have been found.