The CO 2 concentrating mechanism and photosynthetic carbon assimilation in limiting CO 2 : how Chlamydomonas works against the gradient

Summary The CO 2 concentrating mechanism ( CCM ) represents an effective strategy for carbon acquisition that enables microalgae to survive and proliferate when the CO 2 concentration limits photosynthesis. The CCM improves photosynthetic performance by raising the CO 2 concentration at the site of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco), simultaneously enhancing carbon fixation and suppressing photorespiration. Active inorganic carbon (Ci) uptake, Rubisco sequestration and interconversion between different Ci species catalyzed by carbonic anhydrases ( CA s) are key components in the CCM , and an array of molecular regulatory elements is present to facilitate the sensing of CO 2 availability, to regulate the expression of the CCM and to coordinate interplay between photosynthetic carbon metabolism and other metabolic processes in response to limiting CO 2 conditions. This review intends to integrate our current understanding of the eukaryotic algal CCM and its interaction with carbon assimilation, based largely on Chlamydomonas as a model, and to illustrate how Chlamydomonas acclimates to limiting CO 2 conditions and how its CCM is regulated.