Pyrenoids primarily act as centers of carbon dioxide (CO2) fixation in algae, creating and ensuring a CO2-rich environment. They sometimes assist in the catalyzing of inorganic CO2 into starch. In some types of algae, pyrenoids serve as storage sites for carbohydrates.
Pyrenoids are sub-cellular, proteinaceous micro-compartments that exist within the chloroplast of many algae. They consist of 90 percent RuBisCo (Ribulose-1,5-biphosphate-carboxylase-oxygenase), an enzyme that plays a key role in photosynthesis. Pyrenoids have a plastic structure that allows them to migrate and respond to light within the chloroplast.
Pyrenoids were first discovered in 1803, but scientists did not know a lot about them until the 1970s, when they were successfully isolated from algae. Scientific evidence suggests that pyrenoids are capable of converting dissolved inorganic carbon (DIC) into carbon dioxide in sufficiently large amounts. This plays a key role in aquatic photosynthesis, where algae find it more difficult to access CO2 because of its extremely slow diffusion rate in water compared to air.
A lot is still unknown about pyrenoids beyond their association with carbon fixation. The structure of these bodies varies vastly with different algal species. For instance, in dinoflagellates, multiple pyrenoids can be observed in a chloroplast, while in other species such as the red algae, a single pyrenoid exists in each chloroplast.