The evolution of organisms capable of oxygenic photosynthesis paralleled a long-term reduction in atmospheric CO₂ and the increase in O₂. Consequently, the competition between O₂ and CO₂ for the active sites of RUBISCO ribulose bisphosphate carboxylase/oxygenase became more and more restrictive to the rate of photosynthesis. In coping with this situation, many algae and some higher plants acquired mechanisms that use energy to increase the CO₂ concentrations (CO₂ concentrating mechanisms, CCMs CO₂ concentrating mechanism ) in the proximity of RUBISCO. A number of CCM variants are now found among the different groups of algae. Modulating the CCMs may be crucial in the energetic and nutritional budgets of a cell, and a multitude of environmental factors can exert regulatory effects on the expression of the CCM components. We discuss the diversity of CCMs, their evolutionary origins, and the role of the environment in CCM modulation.

The literature, and previously unpublished data from the authors’ laboratories, shows that the δ13C of organic matter in marine macroalgae and seagrasses collected from the natural environment ranges from –3 to –35‰. While some marine macroalgae have δ13C values ranging over more than 10‰ within the thallus of an individual (some brown macroalgae), in other cases the range within a species collected over a very wide geographical range is only 5‰ (e.g. the red alga Plocamium cartilagineum which has values between –30 and –35‰). The organisms with very negative δ13C (lower than –30‰) are mainly subtidal red algae, with some intertidal red algae and a few green algae; those with very positive δ13C values (higher than –10‰) are mainly green macroalgae and seagrasses, with some red and brown macroalgae. The δ13C value correlates primarily with taxonomy and secondarily with ecology. None of the organisms with δ13C values lower than –30‰ have pyrenoids. Previous work showed a good correlation between δ13C values lower than –30‰ and the lack of CO2 concentrating mechanisms for several species of marine red algae. The extent to which the low δ13C values are confined to organisms with diffusive CO2 entry is discussed. Diffusive CO2 entry could also occur in organisms with higher δ13C values if diffusive conductance was relatively low. The photosynthesis of organisms with δ13C values more positive than –10‰ (i.e. more positive than the δ13C of CO2 in seawater) must involve HCO3- use.

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 .