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. 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 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 (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. In the following half-century, phycologists often used the pyrenoid as a taxonomic marker, but physiologists long failed to appreciate the importance of pyrenoids in aquatic photosynthesis. The classical paradigm, which prevailed until the early 1980s, was that the pyrenoid was the site of starch synthesis. Microscopic observations were easily misleading as a starch sheath often encloses pyrenoids. The discovery of pyrenoid deficient mutants with normal starch grains in the green alga Chlamydomonas reinhardtii, as well as starchless mutants with perfectly formed pyrenoids, eventually discredited this hypothesis. It was not before the early 1970s that the proteinaceous nature of the pyrenoid was elucidated, when pyrenoids were successfully isolated from a green alga, and showed that up to 90% of it was composed of biochemically active RuBisCO. In the following decade, more and more evidence emerged that algae were capable of accumulating intracellular pools of DIC, and converting these to CO2, in concentrations far exceeding that of the surrounding medium. Badger and Price first suggested the function of the pyrenoid to be analogous to that of the carboxysome in cyanobacteria, in being associated with CCM activity. CCM activity in algal and cyanobacterial photobionts of lichen associations was also identified using gas exchange and carbon isotope isotopes and associated with the pyrenoid by Palmqvist and Badger et al. The Hornwort CCM was later characterized by Smith and Griffiths.