Carbon acquisition by diatoms

Diatoms are responsible for up to 40% of primary productivity in the ocean, and complete genome sequences are available for two species. However, there are very significant gaps in our understanding of how diatoms take up and assimilate inorganic C. Diatom plastids originate from secondary endosymbiosis with a red alga and their Form ID Rubisco (ribulose-1,5-bisphosphate carboxylase-oxygenase) from horizontal gene transfer, which means that embryophyte paradigms can only give general guidance as to their C acquisition mechanisms. Although diatom Rubiscos have relatively high CO₂ affinity and CO₂/O₂ selectivity, the low diffusion coefficient for CO₂ in water has the potential to restrict the rate of photosynthesis. Diatoms growing in their natural aquatic habitats operate inorganic C concentrating mechanisms (CCMs), which provide a steady-state CO₂ concentration around Rubisco higher than that in the medium. How these CCMs work is still a matter of debate. However, it is known that both CO₂ and HCO₃⁻ are taken up, and an obvious but as yet unproven possibility is that active transport of these species across the plasmalemma and/or the four-membrane plastid envelope is the basis of the CCM. In one marine diatom there is evidence of C₄-like biochemistry which could act as, or be part of, a CCM. Alternative mechanisms which have not been eliminated include the production of CO₂ from HCO₃⁻ at low pH maintained by a H⁺ pump, in a compartment close to that containing Rubisco.