## 2 Carotenoids  
### Characteristics and Biological Function of Carotenoids  
Carotenoids are one of the isoprenoid derivatives with distinctive colors formed by six conjugated double bonds in their polyene chains, each double bond being either _truns_ or _cis_ and containing 40 carbon atoms (tetraterpenes) [20; 21] and formed by head-to-head association [22]. Carotenoids are typically found in _trams_ form in nature [23; 24], but can be commercialized in _cis_ form due to the influence of environmental factors such as temperature and light [25]. Carotenoids are also easily degraded under the influence of oxygen [24]. In general, carotenoids are categorized into carotenes in the form of unsaturated hydrocarbons (Figure 2), such as \(\alpha\)-carotene, \(\beta\)-carotene, \(\gamma\)-carotene, \(\delta\)-carotene, and phytoene as well as xanthophylls containing oxygen with substituents of hydroxyl groups, keto, epoxy, and aldehydes consisting of lutein, zeaxanthin, neoxanthin, violaxanthin, and astaxanthin [26]. \(\alpha\)-carotene and \(\beta\)-carotene are isomers that have the same molecular formula (C\({}_{40}\)H\({}_{56}\)). Lycopene is a symmetrical tetraterpene that can undergo isomerization to any of a number of _cis_-isomers when exposed to heat [27]. Cyclization of lycopene cyclase epsilon forms \(\gamma\)-carotene [28]. In nature, lutein (C\({}_{40}\)H\({}_{64}\)O\({}_{2}\)) and its stereoisomer zeaxanthin coexist with all _trans_-lutein as the most common geometric isomer [29]. When exposed to excess light, zeaxanthin itself can be formed from de-epoxidized violaxanthin [30]. Similar to lutein, neoxanthin (C\({}_{40}\)H\({}_{64}\)O\({}_{4}\)) is often present in all _trans_- or 9 _cis_-isomer forms [31]. Cryptoxanthin (C\({}_{40}\)H\({}_{64}\)O\({}_{4}\)) is an oxygenated carotenoid with one hydroxyl group molecule and is classified as a major xanthophyll carotenoid [32]. At least 1200 types of carotenoids have been discovered in 722 organisms [33]. Carotenoid pigments, which range in color from yellow to red, can be synthesized by plants, algae, fungi, and bacteria. Each organism can synthesize different types of carotenoids, depending on the precursors, metabolic pathways, gene clusters, and the needs of each species. As a terpene compound, the raw material for carotenoids is C5 in the form of isopentenyl pyrophosphate (IPP) and its isomer dimethylallyl pyrophosphate (DMAPP), which are synthesized from the mevalonate (MVA) and non-mevalonate (MEP) pathways [34; 35]. The  
Figure 2: Various carotenoid structures. The basic structure of carotenoids is a tetraterpene. Carotenoid variations result from the presence of various side groups and different double-bond forms. (**a**) Carotene group; (**b**) xanthophyll group. formation of geranyl pyrophosphate (GPP) and elongation of farnesyl diphosphate (FPP), which is then converted to geranylgeranyl pyrophosphate (GGPP), occurs as a result of head-tail condensation of IPP and DMAPP. Phytoene has the molecular formula C40H64 and is formed from two molecules of GGPP. Phytoene undergoes desaturation to form lycopene. Lycopene can further be converted to \(\beta\)-carotene via lycopene cyclase [36]. \(\beta\)-carotene hydroxylase will convert \(\beta\)-carotene into zeaxanthin, while if \(\beta\)-carotene ketolase and \(\beta\)-carotene hydroxylase are working, then \(\beta\)-carotene will be converted to astaxanthin [37]. The entirety of the carotenoid biosynthesis pathway can be visualized in Figure 3.  
Under conditions of high environmental stress such as high salinity, high temperature, and nitrogen deficiency, the microalga _Haematococcus pluvialis_ accumulates astaxanthin in an effort to protect and adapt [38]. Meanwhile, _Monascus purpureus_ YY-1 upregulates carotenoid biosynthesis as a result of carbon deficiency [39]. Cyanobacteria, which are  
Figure 3: Carotenoid biosynthesis pathway. Various kinds of carotenoid derivatives are synthesized based on the genes contained in the carotenoid gene cluster. MVA and MEP pathways are shown in green.