Pyrenoid

Pyrenoids are sub-cellular phase-separated micro-compartments found in chloroplasts of many algae,[1] and in a single group of land plants, the hornworts.

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).

[7] From these pioneering observations, Mereschkowski eventually proposed, in the early 20th century, the symbiogenetic theory and the genetic independence of chloroplasts.

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.

Early studies suggested that RuBisCO is arranged in crystalline arrays in the pyrenoids of the diatom Achnanthes brevipes[21] and the dinoflagellate Prorocentrum micans.

[35] The confinement of the CO2-fixing enzyme into a subcellular micro-compartment, in association with a mechanism to deliver CO2 to that site, is believed to enhance the efficacy of photosynthesis in an aqueous environment.

The molecular basis of the pyrenoid and the CCM have been characterised to some detail in the model green alga Chlamydomonas reinhardtii.

Pyrenoids are highly plastic structures and the degree of RuBisCO packaging correlates with the state of induction of the CCM.

Induction and regulation of the Chlamydomonas CCM was recently studied by transcriptomic analysis, revealing that one out of three genes are up- or down-regulated in response to changed levels of CO2 in the environment.

With the rise of large terrestrial based flora following the colonisation of land by ancestors of Charophyte algae, CO2 levels dropped dramatically, with a concomitant increase in O2 atmospheric concentration.

Predictions of past CO2 levels suggest that they may have previously dropped as precipitously low as that seen during the expansion of land plants: approximately 300 MYA, during the Proterozoic Era.

Evidence of multiple gains and losses of pyrenoids over relatively short geological time spans was found in hornworts.