Adapting from Low to High: An Update to CO(2)-Concentrating Mechanisms of Cyanobacteria and Microalgae

The intracellular accumulation of inorganic carbon (C(i)) by microalgae and cyanobacteria under ambient atmospheric CO(2) levels was first documented in the 80s of the 20th Century. Hence, a third variety of the CO(2)-concentrating mechanism (CCM), acting in aquatic photoautotrophs with the C(3) photosynthetic pathway, was revealed in addition to the then-known schemes of CCM, functioning in CAM and C(4) higher plants. Despite the low affinity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) of microalgae and cyanobacteria for the CO(2) substrate and low CO(2)/O(2) specificity, CCM allows them to perform efficient CO(2) fixation in the reductive pentose phosphate (RPP) cycle. CCM is based on the coordinated operation of strategically located carbonic anhydrases and CO(2)/HCO(3)(−) uptake systems. This cooperation enables the intracellular accumulation of HCO(3)(−), which is then employed to generate a high concentration of CO(2) molecules in the vicinity of Rubisco’s active centers compensating up for the shortcomings of enzyme features. CCM functions as an add-on to the RPP cycle while also acting as an important regulatory link in the interaction of dark and light reactions of photosynthesis. This review summarizes recent advances in the study of CCM molecular and cellular organization in microalgae and cyanobacteria, as well as the fundamental principles of its functioning and regulation.