The ecology and evolution of single-cell C₄-like photosynthesis in diatoms:Relevance to C₄ rice

The 10,000 or more species of diatoms are microscopic photosynthetic organisms of the class Bacillariophyceae in the phylum H+eterokontophyta. They are dominant primary producers in marine and inland water habitats, and may account for up to 20% of global primary productivity. The core carboxylation enzyme in their photosynthesis is Form ID Rubisco (ribulose bisphosphate carboxylase-oxygenase), which, if it replaced rice Form IB Rubisco on a molecule- for-molecule basis, would give slightly lower rates of photosynthesis at extant CO2 concentrations. These kinetic characteristics, along with the low conductance for CO2 of aqueous boundary layers, rationalize the occurrence of CCMs (inorganic carbon-concentrating mechanisms) in all diatoms investigated. It was assumed that these mechanisms, which increase the CO2 concentration around Rubisco, were all based on active transport of CO2, H+CO3 -, or H++ across membranes. It now appears, from recent extensions of earlier work, that there is a C4-like photosynthetic carbon metabolism in certain diatoms. H+owever, more work is needed to determine the extent to which diatoms have photosynthesis analogous to that of single-cell C4 higher plants. The relevance of this work to producing C4 rice probably comes more from concepts than from the direct introduction of diatom genes in rice. One such concept is the possibility that C4-like photosynthesis in diatoms involves no carbonic anhydrases (CAs), and so needs less Zn. H+owever, this requires H+CO3 - entry, so decreased Zn costs of growth may be less readily achieved in rice unless phosphoenolpyruvate carboxykinase (using CO2) replaces phosphoenolpyruvate carboxylase (using H+CO3 -) as the C4 carboxylase.