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 (CA^s), 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.