THE CO2 CONCENTRATING MECHANISM IN CYANOBACTERIA AND MICROALGAE

Over the past 10 years it has become clear that cyanobacteria and microalgae possess mechanisms for actively acquiring inorganic carbon from the external medium and are able to use this to elevate the CO2 concentration around the active site of the primary photosynthetic carboxylating enzyme, ribulose bisphosphate carboxylase-oxygenase (Rubisco). This results in a vastly enhanced photosynthetic affinity for inorganic carbon (C(i)) and improved photosynthetic efficiency. The CO2 concentrating mechanism is dependent on the existence of membrane bound C(i) transport systems, and a microenvironment within the cell where the accumulated C(i) can be used to elevate CO2 at the site of Rubisco. Evidence presented in this review suggests that in cyanobacteria this is achieved by the packaging of Rubisco and carbonic anhydrase (CA) into discrete structures, which are termed carboxysomes. Analogous structures in microalgae, termed pyrenoids, may perform a similar function. The recovery and analysis of high-CO2-requiring mutants has greatly advanced our understanding of the mechanisms and genes underlying these systems, especially in cyanobacteria, and this review places particular emphasis on the contribution made by molecular genetic approaches.