## 3 Carotenoids in the Context of Life  
Carotenoids have received much attention because of their various and important biological roles in all living systems [4, 69, 70, 71]. Although some of those biological roles have been already mentioned in the previous section, this Section includes details about carotenoids biological roles.  
In most of the organisms, the most relevant biological functions of carotenoids are linked to their antioxidant properties, which directly emerge from their molecular structure. Xanthophyll carotenoids in particular are free radical scavengers, potent quenchers of reactive oxygen species (ROS) and nitrogen oxidative species (NOS), and chain-breaking antioxidants. Asthaxanthin and canthaxantin, for example, are better antioxidants and scavengers of free radicals than \(\beta\)-carotene. In recent years, the understanding of ROS-induced oxidative stress mechanisms and the search for suitable strategies to fight oxidative stress has become one the major goals of medical research efforts [3]. On the other hand, carotenoid pigments are one of these natural products responsible for colours: yellow, orange, red, and purple colours in a wide variety of plants, animals, and microorganisms are due to those compounds [72].  
In animals and humans, these compounds are precursors of vitamin A (provitamin A activity) and retinoid compounds required for morphogenesis and embryonic development [35, 73]. Vitamin A is well recognized as a factor of great importance for child health and survival, its deficiency causes disturbances in vision and various related lung, trachea and oral cavity pathologies. Animals and humans cannot synthesize carotenoids _de novo_, although are able to convert them into vitamin A. Diet is the only source for these precursors for retinol synthesis, fruits, vegetables and microalgae being the major suppliers of provitamin A active carotenoids [3, 35]. Other biological roles and functions of carotenoids in these organisms include: absorbers of light energy, oxygen transporters, scavengers of active oxygen [2, 74], antitumor and enhancers of _in vitro_ antibody production [75, 76]. In birds and fish, carotenoids are an important signal of good nutritional condition and are used in ornamental displays as a sign of fitness and to increase sexual attractiveness [71, 77].  
In algae and higher plants, carotenoids serve as regulators of plant growth and development, as accessory pigments in photosynthesis and as a photoprotectors. Thus, they contribute to light harvesting, maintaining the structure and function of photosynthetic complexes, quenching chlorophyll triplet states, scavenging ROS, and dissipating excess energy [34]. On the other hand, carotenoids are also precursors for the hormones abscisic acid (ABA) and strigolactones, and as attractants for other organisms, such as pollinating insects and seed-distributing herbivore [35]. In plants, those pigments are involved in various biological processes, such as photosynthesis, hormones synthesis, photomorphogenesis, photoprotection and development [4]. Apart from these important roles, due to their striking and rich colour, carotenoids are important floral pigments serving to attract pollinators and seed dispersers.  
Finally, microorganisms are a great source of diverse carotenoids. As mentioned before for other organisms, in microorganisms, carotenoids are in charge of light protection, cell colour and antioxidative stress mechanisms. It is important to highlight that some carotenoids, such as salinixanthin or thermozeaxanthin, are only produced by some extremophilic microorganisms [78, 79]. During the last 30 years, researchers, as well as research and development companies, have paid attention to microorganisms due to the high capability of carotenoid production that some species exhibit. This fact, coupled with new insights on molecular biology techniques and downstream process make those microorganisms good sources for carotenoids production at large scale.