CCM in photosynthetic bacteria and marine alga

The evolution of organisms capable of oxygenic photosynthesis paralleled a long-term reduction in atmospheric CO2 and the increase in O2. Consequently, the competition between O2 and CO2 for the active sites of RUBISCO became more and more restrictive to the rate of photosynthesis. In coping with this situation, many algae and some higher plants acquired mechanisms that use energy to increase the CO2 concentrations (CO2 concentrating mechanisms, CCMs) in the proximity of RUBISCO. The CCM improves photosynthetic performance by raising the CO2 concentration at the site of ribulose-1,5-bisphos-phate carboxylase/oxygenase (Rubisco), simultaneously enhancing carbon fixation and suppressing photo-respiration. Active inorganic carbon (Ci) uptake, Rubisco sequestration and inter-conversion between different Ci species catalyzed by carbonic anhydrases (CAs) are key components in the CCM, and an array of molecular regulatory elements is present to facilitate the sensing of CO2 availability, to regulate the expression of the CCM and to coordinate interplay between photosynthetic carbon metabolism and other metabolic processes in response to limiting CO2 conditions. Although the molecular components underpinning the above metabolics is very much clear, the related regulatory pathways still need to be elucidated.