What is "pyrenoid"

Pyrenoid \Py*re"noid\, n. [Gr. ? like a kernel. See Pyrena, and -oid.] (Zo["o]l.) A transparent body found in the chromatophores of certain Infusoria.

n. (context biology English) any of several transparent structures found in the chloroplast of certain algae etc.; they are responsible for the fixation of carbon dioxide and the formation of starch

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 (CCM). Their main function is to act as centres of carbon dioxide (CO) fixation, by generating and maintaining a CO rich environment around the 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 terrestrial habitats, and this has implications for their ability to access CO for photosynthesis. CO 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 and is slow to gain CO from the air. Finally, CO equilibrates with bicarbonate (HCO) 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 HCO. The net result of this is a low concentration of free CO that is barely sufficient for an algal Rubisco to run at a quarter of its maximum velocity, and thus, CO availability may sometimes represent a major limitation of algal photosynthesis.