Carbon concentrating mechanism in cyanobacteria: necessity and evolution

Cyanobacteria are Gram-negative photosynthetic bacteria that produce oxygen similar to eukaryotic algae and higher plants during photosynthesis. These prokaryotes appeared on the Earth when there was no oxygen in the atmosphere, and oxygen produced by them led to the development of present-day aerobic life. However, cyanobacteria lack any subcellular organelles like a nucleus, chloroplast, and mitochondria, and therefore the evolution of the ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) enzyme in the presence of oxygen resulted in the oxygenase activity. Also, CO2 is limiting in aquatic environments where it is readily available in the form of bicarbonate (HCO3¯). Thus the oxygenase activity of Rubisco and availability of HCO3¯ acted as a selection pressure in the cyanobacteria for the evolution of an efficient CO2 concentrating mechanism (CCM) to minimize photorespiration and utilization of available forms of inorganic carbon. In this chapter, we will discuss different components of cyanobacterial CCM such as carboxysome, carbonic anhydrase, and inorganic carbon transporters. We have also included information on evolutionary pressure that resulted in the development of CCM in cyanobacteria.