A structure embedded in or associated with the plastids, involved with fixing carbon dioxide. Assumes many shapes, from spheres to plates to rods. A diatom cell may have several. Visible with transmission electron microscopy, but usually indistinguishable with light microscopy. From Greek for the shape of a nut.

Pyrenoids are composed primarily of the crucial enzyme, rubisco, and are often traversed by thylakoid membranes continuous with stromal thylakoids in the plastids. Pyrenoids concentrate the often low amounts of carbon dioxide in water for use by rubisco, which incorporates carbon dioxide into the first step of carbohydrate production by the Calvin cycle.