Microalgal carbon-dioxide-concentrating mechanisms: Chlamydomonas inorganic carbon transporters

Aquatic photosynthetic micro-organisms have adapted to the variable and often-limiting availability of CO2, and inorganic carbon (Ci) in general, by development of inducible CO2-concentrating mechanisms (CCMs) that allow them to optimize carbon acquisition. Both microalgal and cyanobacterial CCMs function to facilitate CO2 assimilation when Ci is limiting via active Ci uptake systems to increase internal Ci accumulation and carbonic anhydrase activity to provide elevated internal CO2 concentrations through the dehydration of accumulated bicarbonate. These CCMs have been studied over several decades, and details of the cyanobacterial CCM function have emerged over recent years. However, significant advances in understanding of the microalgal CCM have been more recent. With the aid of mutational approaches and the availability of multiple microalgal genome sequences, an integrated picture of the functional components of the microalgal CCMs is emerging, together with the molecular details regarding the function and regulation of the CCM. This review will focus on the recent advances in identifying and characterizing the Ci transport components of the microalgal CCM, especially in the model organism Chlamydomonas reinhardtii Dangeard.