Pyrenoid

pyrenoids
Not to be confused with pyrena, a nutlet within a drupaceous fruit.wikipedia
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Chloroplast

chloroplastschloroplast stromaplastoglobuli
Pyrenoids are sub-cellular micro-compartments found in chloroplasts of many algae, and in a single group of land plants, the hornworts.
Some contain pyrenoids.

Hornwort

Anthocerotophytahornwortsrigid hornwort
Pyrenoids are sub-cellular micro-compartments found in chloroplasts of many algae, and in a single group of land plants, the hornworts.
In half of the roughly 200 hornwort species, this chloroplast is fused with other organelles to form a large pyrenoid that both enables more efficient photosynthesis and stores food.

Photosynthesis

photosyntheticphotosynthesizephotosynthesizing
Their main function is to act as centres of carbon dioxide (CO 2 ) fixation, by generating and maintaining a CO 2 rich environment around the photosynthetic enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO).
Pyrenoids in algae and hornworts also act to concentrate around RuBisCO.

Chlamydomonas reinhardtii

C. reinhardtiiChlamydomonasChlamydomonas reinhardtii''.
In the unicellular red alga Porphyridium purpureum and in the green alga Chlamydomonas reinhardtii, there is a single highly conspicuous pyrenoid in a single chloroplast, visible using light microscopy.
It has a cell wall made of hydroxyproline-rich glycoproteins, a large cup-shaped chloroplast, a large pyrenoid, and an eyespot that senses light.

RuBisCO

rbcLrbc''Lribulose 1,5-bisphosphate carboxylase/oxygenase
Their main function is to act as centres of carbon dioxide (CO 2 ) fixation, by generating and maintaining a CO 2 rich environment around the photosynthetic enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO).
Some plants, many algae, and photosynthetic bacteria have overcome this limitation by devising means to increase the concentration of carbon dioxide around the enzyme, including carbon fixation, crassulacean acid metabolism, and the use of pyrenoid.

Jean Pierre Étienne Vaucher

VaucherVauch.
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.
He is credited for describing the development of the networks that occur in the cells of Hydrodictyon (water net algae), and for describing the pyrenoid of algae.

Carboxysome

carboxysomes
Pyrenoids therefore seem to have a role analogous to that of carboxysomes in cyanobacteria.

Photorespiration

C2 pathwayC2 photosynthesisphotorespiratory
Having a CCM favours carboxylation over wasteful oxygenation by RuBisCO.
In nearly all species of eukaryotic algae (Chloromonas being one notable exception), upon induction of the CCM, ~95% of RuBisCO is densely packed into a single subcellular compartment: the pyrenoid.

Algae

algaalgalfilamentous algae
Pyrenoids are sub-cellular micro-compartments found in chloroplasts of many algae, and in a single group of land plants, the hornworts.

Enzyme

enzymologyenzymesenzymatic
Their main function is to act as centres of carbon dioxide (CO 2 ) fixation, by generating and maintaining a CO 2 rich environment around the photosynthetic enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO).

Cyanobacteria

blue-green algaecyanobacteriumblue green algae
Pyrenoids therefore seem to have a role analogous to that of carboxysomes in cyanobacteria.

PH

pH levelneutralpH value
Finally, CO 2 equilibrates with bicarbonate (HCO 3 − ) when dissolved in water, and does so on a pH-dependent basis.

Enzyme kinetics

kineticsping-pong mechanismenzyme kinetic
The net result of this is a low concentration of free CO 2 that is barely sufficient for an algal RuBisCO to run at a quarter of its maximum velocity, and thus, CO 2 availability may sometimes represent a major limitation of algal photosynthesis.

Andreas Franz Wilhelm Schimper

Andreas SchimperA. F. W. SchimperSchimper
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.

Konstantin Mereschkowski

Konstantin MerezhkovskyMereschkowskiMereschkowsky
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.

Symbiogenesis

endosymbiotic theorysecondary endosymbiosisendosymbiotic
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.

Phycology

phycologistalgologistalgology
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.

Cell membrane

plasma membranemembranecell membranes
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.

Chloroplast membrane

inner chloroplast membraneouter chloroplast membraneChloroplast outer membrane
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.

Thylakoid

thylakoid membranethylakoid membranesthylakoid lumen
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 anhydrase

Carbonic anhydrasescarboanydrasecarbonate dehydratase
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.

Cytoplasm

cytoplasmiccytosolicintracytoplasmic
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.

Stroma (fluid)

stromachloroplast stromastromal proteins
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.

Hypothesis

hypotheseshypotheticalhypothesized
There are several hypotheses as to the origin of pyrenoids.

Charophyceae

CharophyceanCharophycaeacharophycea
With the rise of large terrestrial based flora following the colonisation of land by ancestors of Charophyte algae, CO 2 levels dropped dramatically, with a concomitant increase in O 2 atmospheric concentration.