The green algae CO concentrating mechanism and photorespiration jointly operate during acclimation to low CO.

Due to low availability of CO in aquatic environment, microalgae have evolved a CO concentrating mechanism (CCM). It has long been thought that operation of CCM would suppress photorespiration by increasing the CO 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 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 together with glycolate dehydrogenase down-regulation prevents the toxic accumulation of non-metabolized photorespiratory metabolites, and (iii.) photorespiration is active at low CO 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.