Efficiency of the CO₂-concentrating mechanism of diatoms

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 108; no. 10; pp. 3830 - 3837
• Main Authors: Hopkinson, Brian M, Dupont, Christopher L, Allen, Andrew E, Morel, François M.M
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 08.03.2011; National Acad Sciences
• Series: Inaugural Article
• Subjects: Active transport; Bacillariophyceae; Biological Sciences; Carbon dioxide; Carbon Dioxide - metabolism; Carbonic Anhydrases - metabolism; Cell Membrane Permeability; Cell membranes; Chloroplasts; Cytoplasm; Diatoms; Diatoms - enzymology; Diatoms - metabolism; Diffusion; Enzymes; Modeling; Photosynthesis; Plant cells
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1018062108

Diatoms are responsible for a large fraction of CO₂ export to deep seawater, a process responsible for low modern-day CO₂ concentrations in surface seawater and the atmosphere. Like other photosynthetic organisms, diatoms have adapted to these low ambient concentrations by operating a CO₂ concentrating mechanism (CCM) to elevate the concentration of CO₂ at the site of fixation. We used mass spectrometric measurements of passive and active cellular carbon fluxes and model simulations of these fluxes to better understand the stoichiometric and energetic efficiency and the physiological architecture of the diatom CCM. The membranes of diatoms are highly permeable to CO₂, resulting in a large diffusive exchange of CO₂ between the cell and external milieu. An active transport of carbon from the cytoplasm into the chloroplast is the main driver of the diatom CCM. Only one-third of this carbon flux is fixed photosynthetically, and the rest is lost by CO₂ diffusion back to the cytoplasm. Both the passive influx of CO₂ from the external medium and the recycling of the CO₂ leaking out of the chloroplast are achieved by the activity of a carbonic anhydrase enzyme combined with the maintenance of a low concentration of HCO₃⁻ in the cytoplasm. To achieve the CO₂ concentration necessary to saturate carbon fixation, the CO₂ is most likely concentrated within the pyrenoid, an organelle within the chloroplast where the CO₂-fixating enzyme is located.