Carotenoids are organic pigments produced in plants, algae, bacteria and fungi. There are over 1100 known carotenoids that can be split into two classes, xanthophylls (which contain oxygen) and carotenes (which are purely hydrocarbons, and contain no oxygen). Carotenoids have two key roles in photosynthesis in plants and algae – light absorption and protection of chlorophyll from photodamage. Carotenoids that contain unsubstituted beta-ionone rings (including beta-carotene, alpha-carotene, beta-cryptoxanthinand gamma-carotene) have vitamin A activity (meaning that they can be converted into retinol). Carotenoids can also act as antioxidants. Lutein, meso-zeaxanthin, and zeaxanthin are present in retina as macular pigments whose importance in visual function remains under clinical research.