Overview

Carotenoids are a widespread class of organic pigments found in plants, algae, some bacteria and fungi. They are tetraterpenoid compounds built from isoprenoid units and are typically yellow, orange or red. In nature they give color to flowers, fruits, leaves in autumn, and many microorganisms. Carotenoids contribute to photosynthetic light harvesting and protect tissues from light-induced damage. For a concise introduction see basic carotenoid concepts and more on their chemical nature at pigment chemistry.

Structure and classification

Molecularly, most carotenoids are C40 hydrocarbons with a system of conjugated double bonds that absorbs visible light. Two major classes are recognized: carotenes, which are hydrocarbons (for example beta-carotene and lycopene), and xanthophylls, which contain oxygenated functional groups such as hydroxyl or keto groups (for example lutein and zeaxanthin). Structural overviews and formal nomenclature are discussed in texts at structural overview and classification notes.

Biosynthesis, occurrence and evolution

Carotenoids are synthesized by the isoprenoid pathway in chloroplasts and chromoplasts of plants and in many algae and microbes. Although most animals cannot synthesize carotenoids de novo and obtain them from diet, a few arthropods (notably some aphids and mites) carry genes enabling carotenoid biosynthesis; these genes are thought to have been acquired by horizontal gene transfer from fungi. Broader distribution across life and evolutionary perspectives are treated in reviews at distribution across life and horizontal gene transfer, with specific exceptional cases summarized at exceptional biosynthesis cases.

Biological functions

Carotenoids serve two principal roles in photosynthetic organisms: they extend the range of wavelengths that can be captured for photosynthesis by transferring absorbed energy to chlorophyll, and they protect the photosynthetic machinery by quenching singlet oxygen and dissipating excess excitation energy as heat. Outside photosynthesis, carotenoids act as antioxidants, stabilize membranes and contribute to pigmentation used in signaling and camouflage. For summaries of photoprotective and antioxidant roles see photoprotective functions and antioxidant activity.

Nutrition and human health

Several carotenoids are nutritionally important. Provitamin A carotenoids such as beta-carotene can be enzymatically converted to retinol (vitamin A) in the human body and support vision, growth and immune function; more on provitamin A and nutrition is at nutrition and provitamin A. Other carotenoids, notably lutein, zeaxanthin and astaxanthin, are concentrated in the retina and macula and absorb high-energy blue and near-ultraviolet light, where they may help protect against light-related damage; see research overviews at eye health research. Foods rich in carotenoids include carrots, sweet potatoes, leafy greens, tomatoes, red peppers, certain algae and seafoods that accumulate pigment; see dietary guides at dietary sources.

Applications and safety

Carotenoids are used commercially as natural colorants in foods, beverages and cosmetics, and as nutritional supplements. Some are produced by microbial fermentation or extracted from plant or algal sources for use in aquaculture feeds and human supplements. Regulatory and safety information is available from food additive and supplement authorities; general guidance is provided at food additive guidance, supplement regulation and consumer information. When considering supplements, consult health professionals and regulatory sources because efficacy and safety can vary by compound and dose.

Analytical methods and research directions

Analysis of carotenoids uses chromatography and spectrophotometry to separate and quantify individual pigments and their isomers. Current research explores roles of carotenoids in human health beyond vision, their function in stress tolerance in plants and algae, biofortification of staple crops to improve provitamin A supply, and sustainable production methods using biotechnology. For further reading, use the topic links above and authoritative reviews in plant biochemistry and nutrition science.

  • Key types: carotenes (hydrocarbon) and xanthophylls (oxygenated).
  • Main functions: light harvesting, photoprotection, antioxidant activity, and pigmentation.
  • Human relevance: some are provitamin A; others support eye health and are used as supplements or colorants.