Carotenoids represent one of the most widely distributed and structurally diverse classes of natural pigments, with important functions in photosynthesis, nutrition, and protection against photooxidative damage. In the eubacterial community, yellow, orange, and red carotenoids are produced by anoxygenic photosynthetic bacteria, cyanobacteria, and certain species of nonphotosynthetic bacteria. Many eukaryotes, including all algae and plants, as well as some fungi, also synthesize these pigments. In noncarotenogenic organisms, such as mammals, birds, amphibians, fish, crustaceans, and insects, dietary carotenoids and their metabolites also serve important biological roles. Within the last decade, major advances have been made in the elucidation of the molecular genetics, the biochemistry, and the regulation of eubacterial carotenoid biosynthesis. These developments have important implications for eukaryotes, and they make increasingly attractive the genetic manipulation of carotenoid content for biotechnological purposes.

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Carotenoids occur primarily in photosynthetic organisms but sometimes in nonphotosynthetic bacteria and eukaryotes as well. The carotenogenic organisms that predominantly carried out anoxygenic photosynthesis were eubacteria and cyanobacteria.

Red and green algae, and terrestrial plants acquired stable endosymbionts complement carotenoids to prokaryotes, other fungi, and other algae, which have a diverse spectrum of pigments. Though, carotenoids cannot be synthesized by animals, they are gaining a vital role in protecting against damage caused by singlet oxygen thereby preventing chronic human diseases. The biological roles of carotenoids in chronic diseases and the growth of aquaculture in the last decade have increased the carotenoids market demand.

Carotenoids- Distribution and Application

Carotenoids occur in nature showing diversity in distribution, function, and structure. Carotenoids primarily function as protectants against porphyrin-mediated photo- oxidations and as accessory pigments (light harvesting) in photosynthetic organisms. With its specific catalytic ability, carotenoids can quench singlet oxygen (O 2) that is generated from triplet-state porphyrins reaction or from metabolism. In nonphotosynthetic organisms, carotenoids sequester species with toxic oxygen from the environment or from metabolism. Just like sterols and hormones, carotenoids occurred in some organisms by necessity and in other organisms by gratuity.

Initially, carotenoids were synthesized by oxygenic photosynthetic prokaryotes and anoxygenic phototropic bacteria. In eukaryotes, carotenoids synthesis was acquired with eukaryotic chloroplasts or by bacterial symbiosis. In green and red algae and terrestrial plants, stable symbiosis was established with prochlorophytes or cyanobacteria. Though, carotenoids cannot be synthesized by animals, carotenes can be modified in some animals by isomerization or by oxidation or by converting to retinoid and Vitamin A. occur primarily in photosynthetic organisms.

Carotenoids- Biological Functions

Carotenoids have many health benefits in humans. They delay or prevent certain chronic diseases including cataracts, cancer, arteriosclerosis, and other maladies.

Further, carotenoids provide as good feed supplements for the poultry industry and in the aquaculture of crustaceans and fishes. Apart from providing nutrition and disease resistance, carotenoids give aesthetic value and brilliant pigmentation to animals, crustaceans, and birds. These developments have significantly contributed to the medical and industrial interests of carotenoids. Traditionally, carotenoids for food and agriculture uses were generated by extracting sources from natural plants (example, corn or marigold flowers) and by chemical synthesis. Microbial synthesis offers a substantial portion of pigments that are used in marine aquaculture and terrestrial agriculture and as potential human nutrients or 'nutraceuticals'.

Carotenoids- Disease Prevention in Humans

In the past 10 years, interest in carotenoids has considerably risen due to the growing evidence of health benefits to humans and also in certain areas including the poultry industry, agriculture, and aquaculture. In the area of human benefits, carotenoids propose several protective roles in certain chronic diseases. For example, carotenoids are said to play an important role in the prevention of cancer and other degenerative diseases including immune-system decline, cancer, cataracts, cardiovascular disease, arthritis, and brain dysfunction. According to research, life expectancy has a relation to oxidative damage of proteins, lipids, and DNA, where carotenoids and other antioxidants play an important role in preventing these oxidations.

The basal metabolic rate also plays a vital role in prolonging human life since metabolism generates oxidative by-products that cause significant damage to cellular components. The oxidants that are primarily generated during metabolism are superoxide, lipid epoxides, hydroxyl radical, hydroperoxides, lipid alkoxyl, and singlet oxygen (O 2). Antioxidant defenses against such damaging agents include carotenoids, ascorbate, and tocopherol.

Carotenoids were the first compounds to effectively quench singlet oxygen. β-carotene was proposed as an important dietary anti-carcinogen by scientists with evidence proving that β-carotene has the ability to prevent cancers caused by viruses and chemicals. Few studies also proved that α-carotene had better effects than β- carotene in preventing skin and lung carcinogenesis in mice. Recently, it was also shown that astaxanthin effectively prevented carcinogenesis of the urinary bladder. Further, carotenoids prevent oxidation of human lipoprotein (LDL).

Carotenoids- Functions in Plants, Animals, and Microorganisms

Carotenoid and its derivatives have several functions in eukaryote's and photosynthetic bacteria. Besides performing the roles of a photoprotectorant and light-harvesting pigments, carotenoids have additional biological functions. For example, in corn coleoptiles, the carotenoid zeaxanthin acts as a receptor for blue light-induced phototropism. Carotenoids are a rich source of hormones and stress response in many plants. In algae, carotenoids are split to retinoid that combine with opsin forming the phototaxis photoreceptor.

Developments are being undertaken to ascertain specific roles of carotenoids pertaining to the medical industry.

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