Pyrenoid

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 (CO₂) fixation, by generating and maintaining a CO₂ 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 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 (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 HCO₃⁻. The net result of this is a low concentration of free CO₂ that is barely sufficient for an algal Rubisco to run at a quarter of its maximum velocity, and thus, CO₂ availability may sometimes represent a major limitation of algal photosynthesis.

Pyrenoids were first described in 1803 by Vaucher (cited in Brown et al.). The term was first coined by Schmitz who also observed how algal chloroplasts formed de novo during cell division, leading Schimper to propose that chloroplasts were autonomous, and to surmise that all green plants had originated through the “unification of a colourless organism with one uniformly tinged with chlorophyll". From these pioneering observations, Mereschkowski eventually proposed, in the early 20th century, the symbiogenetic theory and the genetic independence of chloroplasts.

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