Update: carbonic anhydrases in plants, algae, and cyanobacteria

This update discusses the physiological roles of carbonic anhydrase (CA) in plants and algae. It examines some new observations about the functions and assumed functions of CA in plants and algae. Specifically, CA does not regulate pH within plant cells as in blood, which has a high CO2 concentration. Nor do the chloroplast CAs help deliver CO2 to Rubisco during C3 photosynthesis in plants. However, CAs are essential to plants and algae. An under-appreciated function of CA is supplying bicarbonate for many other carboxylases in plants and algae. These CA-requiring carboxylases are involved in nucleotide, fatty acid, and amino acid biosynthesis. In addition, CAs are also essential to the operation of CO2 concentrating mechanisms in C4 plants and algae. The functions of the various CAs in C4 photosynthesis and in the CO2 concentrating mechanisms of cyanobacteria and eukaryotic algae are reviewed in detail.