The evolution of inorganic carbon concentrating mechanisms in photosynthesis

Inorganic carbon concentrating mechanisms (CCMs) catalyse the accumulation of CO 2 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 , CO 2 and/or H + , 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 CO 2 but also photosynthetically active radiation, nitrogen, phosphorus and iron. The evolution of CCMs is generally considered in the context of decreased CO 2 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 CO 2 availability to cells and increase the O 2 concentration within them, inhibiting rubisco and generating reactive oxygen species, including O 3 .