The evolution of inorganic carbon concentrating mechanisms in photosynthesis

Inorganic carbon concentrating mechanisms (CCMs) catalyse the accumulation of CO2 around rubisco in all cyanobacteria, most algae and aquatic plants and in C-4 and crassulacean acid metabolism ( CAM) vascular plants. CCMs are polyphyletic ( more than one evolutionary origin) and involve active transport of HCO3-, CO2 and/or H C, or an energized biochemical mechanism as in C-4 and CAM plants. While the CCM in almost all C-4 plants and many CAM plants is constitutive, many CCMs show acclimatory responses to variations in the supply of not only CO2 but also photosynthetically active radiation, nitrogen, phosphorus and iron. The evolution of CCMs is generally considered in the context of decreased CO2 availability, with only a secondary role for increasing O-2. However, the earliest CCMs may have evolved in oxygenic cyanobacteria before the atmosphere became oxygenated in stromatolites with diffusion barriers around the cells related to UV screening. This would decrease CO2 availability to cells and increase the O-2 concentration within them, inhibiting rubisco and generating reactive oxygen species, including O-3.