The functioning of the CO₂ concentrating mechanism in several cyanobacterial strains: A review of general physiological characteristics, genes, proteins, and recent advances

Cyanobacteria (blue-green algae) possess an environmental adaptation for survival at low CO₂ concentrations. The adaptation is known as a CO₂ concentrating mechanism (CCM), and it functions to actively transport and accumulate inorganic carbon (HCO₃^- and CO₂; C(i)) within the cell and then uses this C(i) pool to provide elevated CO₂ concentrations around the primary CO₂-fixing enzyme, ribulose hisphosphate carboxylase-oxygenase (Rubisco). It appears that the site of CO₂ elevation is within a unique microcompartment known as the carboxysome, which is a proteinaceous polyhedral body that contains most, if not all, of the Rubisco within the cell. This review covers comparative aspects of physiology, genetics, and proteins involved in the cyanobacterial CCM with particular focus on recent advances. This review highlights information on three strains of unicellular cyanobacteria, namely Synechocystis PCC6803 (freshwater strain; for which a full genome database is now available), Synechococcus PCC7002 (coastal marine strain) and Synechococcus PCC7942 (freshwater strain). Genes that may be involved in the CCM, directly or indirectly, are summarized in tabular form. For Synechocystis PCC6803, the number of genes related to CCM activity is now in excess of 50; however, 19 of these components have the potential to code for several distinct type-1, NADH dehydrogenase complexes.