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.