Editing Pyrenoid (section)

{{Short description|Organelle found within the chloroplasts of algae and hornworts}}
{{Hatnote|Not to be confused with [[pyrena]], a nutlet within a drupaceous fruit.}}
[[File:Cross section of a Chlamydomonas reinhardtii algae cell, a 3D representation.jpg|thumb|270px|Cross section of a ''Chlamydomonas reinhardtii'' algae cell, a 3D representation]]

'''Pyrenoids''' are sub-cellular phase-separated micro-compartments found in [[chloroplast]]s of many [[algae]],<ref name = king>Giordano, M., Beardall, J., & Raven, J. A. (2005). CO<sub>2</sub> concentrating mechanisms in algae: mechanisms, environmental modulation, and evolution. ''Annu. Rev. Plant Biol.'', 56, 99-131. {{PMID|15862091}}  {{doi|10.1146/annurev.arplant.56.032604.144052}}</ref> and in a single group of land plants, the [[hornwort]]s.<ref name=bop>Villarreal, J. C., & Renner, S. S. (2012) Hornwort pyrenoids, carbon-concentrating structures, evolved and were lost at least five times during the last 100 million years. ''Proceedings of the National Academy of Sciences'',109(46), 1873-1887. {{PMID|23115334}}</ref><ref>{{cite journal | vauthors=Robison TA, Oh ZG, Lafferty D, Xu X, ((Villarreal JCA)), Gunn LH, Li FW| journal=Nature Plants | title=Hornworts reveal a spatial model for pyrenoid-based CO2-concentrating mechanisms in land plants | pages=1–11 | publisher=Nature Publishing Group | date=3 January 2025 | issn=2055-0278 | doi=10.1038/s41477-024-01871-0}}
</ref> Pyrenoids are associated with the operation of a carbon-concentrating mechanism (CCM). Their main function is to act as centres of carbon dioxide (CO<sub>2</sub>) fixation, by generating and maintaining a CO<sub>2</sub>-rich environment around the [[photosynthesis|photosynthetic]] [[enzyme]] [[ribulose-1,5-bisphosphate carboxylase/oxygenase]] (RuBisCO). 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 CO<sub>2</sub> for photosynthesis. CO<sub>2</sub> 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 CO<sub>2</sub> and is slow to gain CO<sub>2</sub> from the air. Finally, CO<sub>2</sub> equilibrates with bicarbonate ({{chem2|HCO3-}}) 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 (DIC) is mainly found in the form of {{chem2|HCO3-}}. The net result of this is a low concentration of free CO<sub>2</sub> that is barely sufficient for an algal RuBisCO to run at a quarter of its [[enzyme kinetics|maximum velocity]], and thus, CO<sub>2</sub> availability may sometimes represent a major limitation of algal photosynthesis.