Pyrenoids are sub-cellular microcompartments which are found in the chloroplasts of many algae, like Spirogyra and in a single group of land plants, the hornworts. In these algae, pyrenoids probably function to fix carbon. In other algae, pyrenoids are the sites of carbohydrate (typically starch) storage. Starch and iodine react to produce a deep blue- black color, so staining a thin algal preparation with iodine will indicate the presence of pyrenoids.