<jats:p> Inorganic carbon concentrating mechanisms (CCMs) catalyse the accumulation of CO <jats:sub>2</jats:sub> around rubisco in all cyanobacteria, most algae and aquatic plants and in C <jats:sub>4</jats:sub> and crassulacean acid metabolism (CAM) vascular plants. CCMs are polyphyletic (more than one evolutionary origin) and involve active transport of <jats:inline-formula> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2641equ11.gif" /> </jats:inline-formula> , CO <jats:sub>2</jats:sub> and/or H <jats:sup>+</jats:sup> , or an energized biochemical mechanism as in C <jats:sub>4</jats:sub> and CAM plants. While the CCM in almost all C <jats:sub>4</jats:sub> plants and many CAM plants is constitutive, many CCMs show acclimatory responses to variations in the supply of not only CO <jats:sub>2</jats:sub> but also photosynthetically active radiation, nitrogen, phosphorus and iron. The evolution of CCMs is generally considered in the context of decreased CO <jats:sub>2</jats:sub> availability, with only a secondary role for increasing O <jats:sub>2</jats:sub> . 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 <jats:sub>2</jats:sub> availability to cells and increase the O <jats:sub>2</jats:sub> concentration within them, inhibiting rubisco and generating reactive oxygen species, including O <jats:sub>3</jats:sub> . </jats:p>