The ins and outs of CO(2)

It is difficult to distinguish influx and efflux of inorganic C in photosynthesizing tissues; this article examines what is known and where there are gaps in knowledge. Irreversible decarboxylases produce CO(2), and CO(2) is the substrate/product of enzymes that act as carboxylases and decarboxylases. Some irreversible carboxylases use CO(2); others use HCO(3) (–). The relative role of permeation through the lipid bilayer versus movement through CO(2)-selective membrane proteins in the downhill, non-energized, movement of CO(2) is not clear. Passive permeation explains most CO(2) entry, including terrestrial and aquatic organisms with C(3) physiology and biochemistry, terrestrial C(4) plants and all crassulacean acid metabolism (CAM) plants, as well as being part of some mechanisms of HCO(3) (–) use in CO(2) concentrating mechanism (CCM) function, although further work is needed to test the mechanism in some cases. However, there is some evidence of active CO(2) influx at the plasmalemma of algae. HCO(3) (–) active influx at the plasmalemma underlies all cyanobacterial and some algal CCMs. HCO(3) (–) can also enter some algal chloroplasts, probably as part of a CCM. The high intracellular CO(2) and HCO(3) (–) pools consequent upon CCMs result in leakage involving CO(2), and occasionally HCO(3) (–). Leakage from cyanobacterial and microalgal CCMs involves up to half, but sometimes more, of the gross inorganic C entering in the CCM; leakage from terrestrial C(4) plants is lower in most environments. Little is known of leakage from other organisms with CCMs, though given the leakage better-examined organisms, leakage occurs and increases the energetic cost of net carbon assimilation.