Acquisition of Inorganic Carbon by Microalgae and Cyanobacteria

Ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) and the Calvin cycle are the dominant features of inorganic carbon assimilation in all cyanobacteria and microalgae. Rubisco carboxylase shows a relatively low affinity for CO₂ and also has an oxygenase activity. These features can lead to inefficiencies in carbon assimilation, involving the process of photorespiration. However, cyanobacteria and algae possess mechanisms that minimise the effects of unfavourable Rubisco kinetics and photorespiration. These involve evolution of Rubiscos with kinetics that are more favourable to carboxylase activity and/or the presence of mechanisms that increase the concentration of CO₂ at the active site of Rubisco (CO₂ concentrating mechanisms, CCMs). CCMs are mostly based on active transport of HCO₃. In one species of marine diatom, there appears to be a biochemical CCM where single cells show a C₃-C₄ intermediate form of C assimilation. In cyanobacteria HCO₃^- accumulation involves active transport of HCO₃^- at the plasmalemma and/or downhill CO₂ entry with energised conversion of CO₂ to HCO₃^- at the thylakoid membrane, with CO₂ accumulated within carboxysomes that contain all of the cellular Rubisco as well as carbonic anhydrase. In eukaryotic microalgae active HCO₃^- transport occurs at either the plasmalemma or chloroplast envelope or both. In those algae that possess them, CO₂ is ultimately concentrated within Rubisco-containing pyrenoids, though it is important to note that the presence of pyrenoids is not an absolute requirement for CCMs. Algae and cyanobacteria can assimilate inorganic carbon in the dark, a process reflecting the need of cells to replenish the supply of C₄ intermediates of the TCA cycle as they are removed for biosynthesis. A few can also assimilate organic carbon sources, including in some cases by phagotrophy.