page_title: Pyrenoid
section_title: Role of Pyrenoids in the CCM
hierarchical_section_title: Pyrenoid / Role of Pyrenoids in the CCM
caption_attribution_description: English: Figure depicting the currently accepted model for the carbon concentrating mechanism seen in Chlamydomonas reinhardtii.
context_page_description: Pyrenoids are sub-cellular micro-compartments found in chloroplasts of many algae, and in a single group of land plants, the hornworts. Pyrenoids are associated with the operation of a carbon-concentrating mechanism. Their main function is to act as centres of carbon dioxide fixation, by generating and maintaining a CO₂ rich environment around the photosynthetic enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase. Pyrenoids therefore seem to have a role analogous to that of carboxysomes in cyanobacteria.
  Algae are restricted to aqueous environments, even in aquatic habitats, and this has implications for their ability to access CO₂ for photosynthesis. CO₂ diffuses 10,000 times slower in water than in air, and is also slow to equilibrate. The result of this is that water, as a medium, is often easily depleted of CO₂ and is slow to gain CO₂ from the air. Finally, CO₂ equilibrates with bicarbonate when dissolved in water, and does so on a pH-dependent basis. In sea water for example, the pH is such that dissolved inorganic carbon is mainly found in the form of HCO₃⁻.
context_section_description: The confinement of the CO₂-fixing enzyme into a subcellular micro-compartment, in association with a mechanism to deliver CO₂ to that site, is believed to enhance the efficacy of photosynthesis in an aqueous environment. Having a CCM favours carboxylation over wasteful oxygenation by RuBisCO. The molecular basis of the pyrenoid and the CCM have been characterised to some detail in the model green alga Chlamydomonas reinhardtii.
  The current model of the biophysical CCM reliant upon a pyrenoid considers active transport of bicarbonate from the extracellular environment to the vicinity of RuBisCO, via transporters at the plasma membrane, the chloroplast membrane, and thylakoid membranes. Carbonic anhydrases in the periplasm and also in the cytoplasm and chloroplast stroma are thought to contribute to maintaining an intracellular pool of dissolved inorganic carbon, mainly in the form of bicarbonate. This bicarbonate is then thought to be pumped into the lumen of transpyrenoidal thylakoids, where a resident carbonic anhydrase is hypothesised to convert bicarbonate to CO₂, and saturate RuBisCO with carboxylating substrate. It is likely that different algal groups evolved different types of CCMs, but it is generally taken that the algal CCM is articulated around a combination of carbonic anhydrases, inorganic carbon transporters, and some compartment to package RuBisCO.
  Pyrenoids are highly plastic structures and the degree of RuBisCO packaging correlates with the state of induction of the CCM. In Chlamydomonas, when the CCM is repressed, for example when cells are maintained in a CO₂-rich environment, the pyrenoid is small and the matrix is unstructured. In the dinoflagellate Gonyaulax, the localisation of RuBisCO to the pyrenoid is under circadian control: when cells are photosynthetically active during the day, RuBisCO assembles into multiple chloroplasts at the centre of the cells; at night, these structures disappear.
caption: The currently hypothesised composition of the CCM found in Chlamydomonas reinhardtii. 1= Extracellular environment. 2= Plasma membrane. 3= Cytoplasm. 4= Chloroplast membrane.  5= Stroma. 6= Thylakoid membrane. 7= Thylakoid lumen. 8= Pyrenoid.
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