3.3.3. Algae

Little is known for the carotenogenesis pathways among algae, but some are proposed based on the chemical structures of carotenoids (Figure 2). Functionally confirmed enzymes are mainly reported in Chlorophyceae including *Chlorella*, *Chlamydomonas*, *Dunaliella* and *Haematococcus* for CrtB, CrtP, CrtL-b, CrtR-b [46], Zep [48], Vde [49], and CrtW (Table 2). 

In the cell-free preparation of *Amphidinium carterae* (Dinophyta), 14C-labellled zeaxanthin was incorporated into allenic carotenoid of neoxanthin, and then into acetylenic diadinoxanthin and C37 peridinin (Figure 2). In addition, the three carbon atoms of C-13<sup>ȝ</sup>,14<sup>ȝ</sup>,20<sup>ȝ</sup> of peridinin were eliminated from neoxanthin (C-13,14,20) [69,70]. In organic chemistry, the C-7,8 double bond of zeaxanthin can be oxidized to the triple bond (acetylene group) of diatoxanthin [17]. 

Allenic carotenoids are very limited in algae. From their chemical structures, all *trans* neoxanthin might be changed to fucoxanthin, dinoxanthin, peridinin, vaucheriaxanthin and diadinoxanthin, but the pathways and enzymes are still unknown (Figures 1 and 2). 

Under a stressful environment, such as high light, UV irradiation and nutrition stress, some Chlorophyceae, such as *Haematococcus*, *Chlorella* and *Scenedesmus*, accumulate ketocarotenoids, canthaxanthin and astaxanthin, which are synthesized by combining CrtR-b and Ά-carotene ketolase (CrtW, BKT) (Table 2) [53–56,71]. Note that although Ά-carotene ketolase of *Haematococcus* and *Chlorella* were named CrtO at first [53,56], they are CrtW-type not CrtOtype from amino acid sequences (Table 2). 

## *3.4. ΅-Carotene Derivatives and Their Synthesis*

In *Arabidopsis thaliana*, Ά-carotene is hydroxylated mainly by the non-heme di-iron enzymes, BCH1 and BCH2 (CrtR-b), to produce zeaxanthin, while ΅-carotene is mainly hydroxylated by  the cytochrome P450 enzymes, CYP97A3 for the Ά-end group and CYP97C1 for the Ά- and <sup>Ή</sup>end groups, to produce lutein [72]. 

Lutein and its derivatives are found only in Rhodophyta (macrophytic type), Cryptophyta, Euglenophyta, Chlorarachniophyta and Chlorophyta (Table 1), but nothing is known for hydroxylation of ΅-carotene. From the chemical structures of siphonaxanthin [12], loroxanthin, prasinoxanthin and monadoxanthin, it could be considered that they are derived from lutein, but the pathways and enzymes are still unknown (Figures 1 and 2). 
