**1. Introduction**

Carotenoids are naturally occurring terpenoid pigments made of isoprene residues, displaying a conjugated double polyene chain. They are hydrophobic compounds and are usually formed by a skeleton of 40 hydrocarbons, although most of the derivatives present in extremophilic microbes like those belonging to the Archaea domain contain 50 carbon atoms. This chain can be terminated by rings or functional groups with oxygen [1]. These compounds can absorb light mainly in the range of 300–600 nm (they show colors that vary from yellow to red) and are responsible for the characteristic pigmentation shown by some flowers, fungi, and several microorganisms. The ability to absorb light at a certain wavelength is related to the number of functional groups and double bonds present in their structure [2]. They are also involved in many biological roles in all living beings, reporting beneficial actions.

To date, over 1100 natural carotenoids have been described in animals, plants, macroalgae, some fungi, and a wide variety of microorganisms (including extremophiles). Due to the structural diversity of these molecules, there are still several biochemical and physiological functions to be associated with this class of compounds. Accordingly, these characteristics enable wide applicability, which drives the global carotenoid market.

Carotenoids are beneficial for human health; however, humans cannot synthesize carotenoids *de novo*, consequently, they are mainly obtained through the diet. In fact, carotenoids are consistently found in tissues or biological fluids, where they play a beneficial role in decreasing the risk of developing some diseases, such as cancer, eye disorders, and autoimmune or cardiovascular diseases.

During the last half-century, significant advances in the research of carotenoids have been made. As an example, much knowledge on their biosynthesis in plants and microbes has been generated, and there have been important breakthroughs in their production in both conventional and genetically modified organisms, not only in the laboratory but also on a large scale [3, 4].

The aim of this book is to highlight new perspectives and applications of carotenoids, including characterization and isolation of new compounds (including rare carotenoids), their production at a mid-large scale (involving new innovative approaches), and uses of carotenoids in different biotechnological fields, such as food science, biomedicine, and cosmetics.

## **2. Current research on carotenoids**

Most of the recent studies on carotenoids describe the importance, chemical composition, and functioning of carotenoids. Although the research is mainly paying attention to C40 carotenoids, the interest in rare carotenoids, such as marine C50, is increasing due to their significant antioxidant activity [3, 5].

There is also an important research field in which efforts are focused on looking for new natural sources from which natural carotenoids can be isolated. Considering their biological roles (not fully described yet), the potential applications of these molecules are still increasing. Some of the better described biological roles of carotenoids in plants, animals, and microbes are given below:


Among carotenoids, β-carotenes, astaxanthin, canthaxanthin, and lutein are highly marketed because they are part of drugs and cosmetics formulas or can be added to food due to their color or their antioxidant activities. So far, the large-scale production and subsequent commercialization of these compounds are carried out through chemical production, which has advantages and disadvantages compared to natural production. Thus, carotenoids chemically produced are characterized by excellent purity and consistency, with a relatively low production cost. However, the chemical synthesis of carotenoids involves the use of reagents, which are not environmentally friendly. Besides, the synthesis of pigments of greater structural complexity is expensive and highly time consuming. Considering all these disadvantages, the production of natural pigments using cell factories reveals a promising approach to obtaining carotenoids on a large scale [6]. In fact, carotenoids productions based on cell factories and environmentally friendly procedures is one of the fields of research attracting more attention worldwide during the last decade.

Recently, several works stated that carotenoids are also interested in sustainable energy and green electronics, which sheds light on more potential applications for these natural compounds [7].

## **3. Challenges related to carotenoid research and applications**

Considerable progress in carotenoid research has been made to understand the carotenoid metabolism in all life forms, including humans. However, despite
