Pyrenoids are subcellular micro-compartments present in many algae's chloroplasts and in hornworts, a solitary terrestrial plant species.
Pyrenoids are involved in the operation of a carbon-concentration process (CCM).
They operate as carbon dioxide (CO2) fixation centres by creating and maintaining a CO2-rich environment surrounding the photosynthetic enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO).