The word carotenoid is derived from the scientific name of the carrot Daucus carote, identified by Wackenroder in 1831 as the first source of carotene . Carotenoids are one of the classes of natural pigments often found in fruits and vegetables. Also, they are produced by algae, filamentous fungi, yeasts, and bacteria  . These pigments are responsible for the colors bright yellow, orange, and red .
Most of the commercialized carotenoids are from vegetable extraction and chemical synthesis. Some problems in the production and commercialization of these dyes, such as seasonal and geographical variability, cannot be controlled . On the other hand, chemical synthesis is a generally complex process that causes the formation of hazardous and inappropriate waste when disposed of in the environment; also, this is not considered safe for health . This results in general concern, which reverberates in many discussions about the unwanted effects of these artificial pigments and the future impacts on human health  .
Therefore, the synthesis of carotenoids by microorganisms is a viable strategy, which meets the safety marketing appeal and makes it possible to obtain these natural pigments on an industrial scale, quickly and efficiently  . Unlike traditional production, the microbial pathway for the production of carotenoids does not depend on external factors and on the seasonal supply of raw materials, which allows the generation of vast pigment yields with diverse coloring  .
Some microorganisms, known as extremophiles, grow in hostile or extreme environments . They are described as potential producers of natural molecules of biotechnological interest. The Caatinga Domain is a very particular habitat with high temperatures, in which the soil can reach 60˚C in periods of drought, thus characterized as an environment of adverse conditions for the growth of microorganisms   .
Some microorganisms can accumulate carotenoids in response to environmental stress, such as oxidative stress caused by high solar incidence, which contributes to their survival and competitiveness. Although these microorganisms and their ability to synthesize pigments are poorly investigated  , bioprospection of unknown microorganisms, as a source of new carotenoids, has been highlighted as a focus of research in the biotechnological area .
The carotenoids extracted from these organisms stimulate applications in cosmetic products, as they have antioxidant properties, solubility and stability in oil-in-water emulsions, which are attractive for application in sunscreens and other anti-UV products  , including generating a patente . β-carotene, lycopene, and astaxanthin are among the carotenoids described and used as photoprotection due to their antioxidant action, which acts against free radicals induced by UV radiation to protect the skin against erythema  .
In this context, the present work sought to summarize the characteristics of carotenoids, their structures, properties, classifications, sources of obtention, commercialization, and applications. Besides, this review addresses the production of carotenoids from extremophilic microorganisms, especially from the Caatinga Domain, as a promising source of these biomolecules with interesting properties; as well as, they envision their applications in cosmetic products, especially those with UV protection.
2.1. Molecular Structure
Natural lipophilic pigments, known for their diversity of structures, carotenoids originate from the biosynthetic pathway of terpenoids . These compounds present in their basic structure eight isoprenoid units joined in such a way that the molecule has inverted symmetry in the center. They have linear structures that are, in general, derived from a C40H56 acyclic chain with a system of double and single bonds distributed along its length .
This system acts as a chromophore and is one of the most striking features of the structure of carotenoids . The length of the chromophore and the presence or absence of functions determine the absorption spectrum. Also, the color of the molecule, besides giving these pigments a high chemical reactivity, can be easily isomerized and oxidized  .
There were characterized about 500 structures. More than 95% of carotenoids are formed by a chain of 40 carbons, which is known as tetraterpenoids. Also, there are groups formed by 30 and 50 carbon atoms  .
From this diversity of structures, carotenoids naturally have different physical, chemical, functional properties, and even stability . Some of these properties were described by these authors are shown in Figure 1.
2.2. Classification and Diversity of Carotenoids
Carotenoids are classified into two large groups according to the characteristics of their molecular structures:
Carotenes: molecular formula C40H56, which include α, β, γ-carotenes, and lycopene;
Xanthophylls: oxygenated and hydroxylated derivatives of carotenes, which include cryptoxanthin (C40H55OH) and lutein (C40H54(OH)2).
This classification occurs according to the absence or presence of oxygen atoms .
Carotenes are characterized by its composition, only of carbon and hydrogen (pure hydrocarbons), which can be cyclized at one or both molecule extremities, while xanthophylls are oxygen-substituting groups, (hydroxy, methoxy, carboxy, keto and epoxy groups) that generate many structural derivatives and higher polarity when compared to carotenes (Figure 2)   .
Figure 1. Physical, chemical, and biological properties of carotenoids. Source: Kirti et al.  with adaptations.
Figure 2. Structure of the carotenoids: (a) xanthophylls; (b) Carotenes. Source: Valduga et al.  with adaptations.
Isomerization and oxidation of trans carotenoids (stable form in nature) to the cis form (loss of color and activity) can occur with the modification of the basic acyclic structure C40 by hydrogenation, dehydrogenation, cyclization or oxidation, generating great variability of carotenoids    . The most commonly found in nature and food are β-carotene, α-carotene, β-cryptoxanthin, lycopene, lutein, violaxanthin, and zeaxanthin; due to this availability, they are the most studied in terms of biological activities that help in health promotion .
3. Application and Commercialization of Carotenoids in Industrial Products
Commercial production of carotenoids has been developed, since 1954. They have been, since then, used as a food coloring and nutritional supplements. Also, they help to maintain the aromas and vitamins of foods . Besides, they are used in the production of margarine and butter, fruit juices and drinks, soups, dairy products, as well as in fish, desserts, sugar, salad dressings, meats, pasta, eggs, and mayonnaise industries  . These biomolecules are also added in animal feed, in aquaculture, for example, they are sources of pigmentation for fish and crustaceans  . This sector of animal feed is the sector with the highest growth, and it is estimated that it will comprise the largest carotenoid market in 2023, according to the Global Carotenoid Market .
Carotenoids can be used in various ways in the pharmaceutical and medical industry, such as pro-biotic, syrups, antimicrobial agents, for controlling diseases, and in the modulation of immunological reactions. For environmental purposes, they are used as bioindicators, as well as they are applied in the cosmetics industry for the production of protectors, tanners, creams, among other products  .
According to Cutzu et al. , the total value of carotenoids in the global market has an annual growth of 2.3%. According to BCC , in 2017, it was expected to reach $1.5 billion, and forecasts for 2022 will be $2.0 billion. In the period between 2007 and 2017, the value increased twice, since the estimated global scale value in 2007 was 766 million dollars. This growth, in the market, has been driven by the properties of carotenoids, global megatrend of the amplification of carotenoids use, demand for applications, and growing interest in developing processes to efficiently produce these pigments  .
Initially and until 2016, the global manufacturing industry for this group of dyes was dominated by suppliers from Europe, such as the United Kingdom, Switzerland, Germany, France, Italy, the Netherlands, and Spain. However, in recent years, the Asia-Pacific region has been growing and receiving investments and research initiatives for the use of carotenoids . The main industrial producers identified in the global carotenoid market are BASF SE (Germany), Hoffmann-La Roche (Switzerland), Royal DSM NV (Netherlands), Chr. Hansen A/S (Denmark), FMC Corporation (USA), Kemin Industries, Inc. (USA), and Cyanotech Corporation (USA) . They produce and commercialize different carotenoids such as β-carotene, canthaxanthin, astaxanthin, apocarotenoids, and citranaxanthin  .
4. Microbiological Production of Carotenoids
Due to the global demand for natural dyes, there is an increasing preference for pigments of microbial origin, and being produced for the food industry, textile dyeing, pharmaceuticals, and cosmetics . This search has resulted in constant research regarding the production of microbial carotenoids in biotechnological processes   . Carotenoids are synthesized by some bacteria, filamentous fungi, yeasts, and microalgae, which have few commercially exploited species   .
Production of carotenoids by microorganisms demonstrates a safe use. Also, it has been an area of intense investigation since it allows the obtaining of the compounds of natural origin on an industrial scale, which is being considered a promising production strategy  .
This type of production has some advantages, as it is a fast, highly efficient process that can be easily managed during production. At the same time, it is superior in versatility and productivity and is independent of climatic conditions when compared to production from fruits and vegetables  . Finally, it is possible to produce different shades of color with a relatively easy procedure for extracting and separating cell biomass. Also, the microorganisms can grow on cheap substrates  .
To further increase yields and guarantee the production of microbial carotenoids, studies have been carried out on processes to optimizing cultivation conditions, the use of cheap substrates, and genetic engineering studies  .
Despite the large number of microbial carotenoids already identified and characterized, only a few are produced on an industrial scale. Also, the market for carotenoids synthesized by bioprocesses is still a little difficult to estimate. It is due to the lack of statistics on regional low-tech products or because production is dispersed in small companies around the world.
Among the products already available on the market obtained through microbially, there is astaxanthin produced by the yeast Phaffia rhodozyma, the bacterium Agrobacterium auranticum, and the freshwater microalgae Haematococcus pluvialis  , also β-carotene synthesized by Blakeslea trispora and zeaxanthin produced by Flavobacterium. It also stands out the production of β-carotene, torulene, and torularodine by yeasts of the genus Rhodotorula  . Besides, these microbial pigments are considered safe and approved by the Food and Drug Administration (FDA) .
Much research is been directed to find new sources of this group of pigments, which has promising properties, and strategies that can increase the efficiency of these biotechnological systems. So, these dyes are commercially viable and can be expanded in the various industrial sectors  .
5. Biotechnological Potential of the Caatinga Domain
The Caatinga domain has the most extreme meteorological values in Brazil. It has the strongest sunshine, the lowest cloudiness, low annual precipitation rates (below 800 mm), and high daytime temperatures, which can vary between 23˚C and 27˚C, while the soil can reach temperatures close to 60˚C  .
This region is of fundamental importance in the life of local communities. It offers a wide variety of plants that are used for medicinal purposes, and that explicit the potential of this domain for research studies of new bioactive products    .
Among the compounds produced by some plants in the Caatinga and identified by their beneficial biological actions to health, there are carotenoids. In a study by David et al. , there was possible to correlate the antioxidant and cytotoxic activity with the presence of β-carotenes in Caatinga plants. The greatest activities were observed in Passiflora cincinnata, Chamaecrista repens, Rollinia leptopetala, Serjania glabrata, Diospyros gaultheriifolia, and Mimosa ophtalmocentra.
Another source of carotenoids in the Caatinga is carotenogenic microorganisms, but little is known about them since most studies describe the carotenoids synthesized by plants, as well as their respective biological activities. In a group of bacteria, for example, the information obtained is that only 1% of bacteria in this domain (including pigmented and non-pigmented ones) have already been discovered and described . However, after the publication of this percentage, referring to groups of microorganisms in the Caatinga Domain and their carotenogenic potentials, there is a scarcity of these data in literature over the years.
Thus, as it constitutes an environment with interesting biotechnological potential and with little information in the field of microbial bioprospecting, greater attention is needed to the Caatinga and the development of more studies ; since, it presents a vast diversity of microorganisms, little explored and in which new natural carotenoid producers can be discovered.
Microorganisms Isolated from the Caatinga Domain as New Sources of Carotenoids
Microorganisms isolated from environments that present extreme conditions are considered hostile or extreme . To guarantee their survival, they develop biochemical mechanisms and produce metabolites to overcome these unfavorable factors. An example of this efficient strategy is the synthesis of carotenoid pigments to minimize the effects of oxidative damage on cells and other damage caused by high sun exposure   .
The soil of these arid or semi-arid environments, considered as a hostile or extreme environment, is recognized as a rich habitat. It has microorganisms that produce natural molecules with biotechnological potential, whose biological activities are extremely rich and still poorly explored   .
Microorganisms most found in the soil microbiota of this environment belong to the phylum Actinobacteria, which perform essential functions for the ecological balance of this habitat . This group grows in extreme conditions, such as soils with low moisture content, typical of semi-arid regions, as well as being representative in terms of richness and diversity  . They produce pigments of different colors such as carotenoids, which characterize the group and contribute to environmental resistance. They also produce extracellular enzymes and important secondary metabolites  .
In a study by Ramos et al. , the pigment production and chromogenic diversity of soil actinobacteria from the semiarid region of Ceará state were evaluated. It classifies some strains according to color. Also, it suggests the presence of some genera described in the literature as carotenoid producers; characterized by a yellow color, the genera Actinoplanes, Agrococcus, Arsenicicoccus, Cellulomonas, Kocuria, Microbacterium, Rhodococcus, Serinicoccus, Streptomyces were found, and as representatives of the pink color Arthrobacter and Gordonia. It can be seen among these genera, that the majority was responsible for the production of yellow carotenoids. According to Kampe et al.  and Kavamura et al. , this color is characteristic of a microbiota specialized in biomolecules with photoprotective properties. Also, they have structures that contribute to these properties.
As actinobacteria, the presence of some yeasts in the microbiota of these hostile Brazilian environments also stands out. Da Silva Andrade et al. , showed that from the yeast, Rhodotorula glutinis, isolated at the semi-arid Pernambucano soil, β-carotene can be obtained by using economical production conditions. Also, the ability of this carotenoid to inhibit cell growth of tumor cells was evaluated and effective for breast cancer and promyelocytic leukemia.
These data corroborate with the idea that the Caatinga Domain is a potential source of microorganisms that produce promising molecules, especially carotenoids. Also, it will be a region of great interest to biotechnologists, which has prospects for applications in industries.
6. Application of Carotenoids in the Cosmetics Industry
Chemical UV filters do not fully protect the skin, as they absorb, instead of reflecting or spreading all the UV photons that reach the skin. Most of the consequences caused by UVA radiation are related to the generation of Reactive Oxygen Species (ROS) after exposure. Therefore, it is necessary to add antioxidant molecules to sunscreens, especially in topical products, which minimize UV damage to the skin .
Carotenoids have aroused the interest of the cosmetic industries, since they are important natural antioxidants acting in the reduction of the generation of free radicals and, consequently, diminish the photodamage in the skin . Also, they have a promising anti-UV capability, which boosts the development of photoprotective cosmetic formulations . Whereas, it is intended to reduce the concentration of chemical filters or replace them with sunscreens based on natural products .
β-carotene, lycopene, and astaxanthin are among the most described carotenoids and used as photo protectors since they have strong antioxidant properties . β-carotene, known as a precursor to provitamin A, a strong inhibitor of 1O2, and free radical scavenger is strongly associated with its accumulation in the skin after ingestion, and later, its transformation into vitamin A in the body, which helps in the formation of melanin .
Lycopene, a potent antioxidant, has a significant correlation in skin roughness. It suggests that higher levels of skin antioxidants effectively can reduce skin roughness  . Astaxanthin acts as an effective antioxidant against lipid peroxidation and oxidative stress. Also, this property is associated with the prevention of the formation of spots and wrinkles on human skin .
However, the application of these carotenoids at the topical level is still poorly investigated; they are normally produced commercially for oral use. Studies with cosmetics formulations, which use these molecules, demonstrate interesting results as far as oral ingestion, reinforcing the approach of using topical carotenoids to prevent oxidative stress induced by acute UV exposure. Also, it supplies cutaneous levels of antioxidants, among other benefits  .
Carotenoids extracted from microorganisms have been stimulating their applications in formulations of sunscreens and other anti-UV products since they have powerful antioxidant properties, solubility, and stability in oil emulsions. Rare carotenoids with excellent properties can be isolated as well as the carotenoids used traditionally , which makes them attractive even for the production of patents .
In this context, due to the extreme conditions described above, the microorganisms isolated from the Caatinga Domain present themselves as promising sources of carotenoids little studied, which stimulates their use in cosmetic products. Also, the production of these molecules is induced by strong UV exposure, which demonstrates interesting antioxidant and photoprotective properties.
Thus, as future perspectives, bioprospecting research in the Caatinga should be strongly encouraged. It will allow an increase in the number of natural resources of carotenoids, and even investigate those considered rare and poorly described. Therefore, based on these data, actions and strategies for the economic use of these molecules can be defined, with the generation of biotechnological products with applications both in the cosmetics industry and in other industrial sectors; through its technology without harming the natural relationships of the environment and preserving its biodiversity .
The authors are grateful for the financial support of the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior ou Educação (CAPES).
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