## 4 Biosynthesis of Xanthophylls  
All carotenoids are synthetized from a common precursor to all isoprenoids, the isopentenyl pyrophosphate (IPP). The biosynthesis of IPP may generally be performed by one of two different pathways (Figure 1): the mexolonate pathway (MVA) in euglenophytes and the plastidial 2-C-methyl-D-erythriol-4-phosphate (MEP) pathway in Chlorophyceae and Cyanophyceae [42; 67; 96; 98]. The MEP pathway is considered to be the main biosynthetic pathway of carotenoids in several microalgae species such as _Chlorella vulgaris_, _Dunaliella salina_, _Senedesmus_ sp., and _Haematococcus pluvialis_[98; 99]. In this pathway, the carotenoid biosynthesis begins through IPP, which isomerises to dimethylallyl pyrophosphate (DMAPP) by the action of the IPP isomerase. Elongation of the carbon chain took place through continuous head-to-tail condensation of IPP to DMAPP followed by growing of the polyprenyl pyrophosphate chain by the action of the prenyltransferase [55; 100]. The second committing step in carotenoid biosynthesis is the condensation of two molecules of geranyl pyrophosphate (GPP, C10), catalyzed by the GGPP synthase, to yield geranyl-geranyl PP (GGPP, C20) [42]. After that, the colourless C40 carotenoid phytoene is formed through the condensation of two GGPP (C20) molecules by the action of phytoene synthase (PSY) [42]. From this step onwards, phytoene undergoes a series of sequential destinations (four desaturation steps) catalyzed by phytoene desaturase (PDS) and zeta-carotene desaturase (ZDS)leading to the formation of pro-lycopene [42]. Pro-lycopene is then isomerized by a specific carotenoid isomerase (CRTISO) into all-trans-lycopene. At this level, the pathway is divided into two branches. A first branch leads to the formation of \(\beta\)-carotene and its xanthophyll derivatives, such as zeaxanthin and ASX, and the second branch leads to the formation of \(\alpha\)-carotene and lutein in microalgae. The formation of \(\beta\) and \(\varepsilon\) rings at both ends of lycopene results in a yield of \(\alpha\)-carotene under the catalysis of lycopene \(\beta\)-cyclase (LCYB) and \(\varepsilon\)-cyclase (LCYE) respectively, whereas the formation of two \(\beta\)-rings at the two ends of lycopene gives rise to \(\beta\)-carotene catalyzed by LCY-b alone [42]. B-carotene can be further hydroxylated by the \(\beta\)-carotene hydroxylase (CHYB) to give zeaxanthin. The amounts of carotenoids produced by each branch of the pathway are dependent on activities of LCYE and LCYB. \(\alpha\)-carotene hydroxylation is assured by two heme-containing cytochrome P450 monoxygenenses (namely, \(\beta\) carotene and carotene \(\varepsilon\)-hydroxylases), which formslutein. In a first branch, zeaxanthin is transformed into violaxanthin by the enzyme zeaxanthin epoxidase (ZEP), which incorporates two epoxy groups at positions C-5,6 and C-5',6'. Zeaxanthin is also transformed in a second branch intodi-keto carotenoid canthaxanthin, and violaxanthin is converted to ASX by \(\beta\)-carotene ket-tolase (BKT) (Figure 1). The two pathways zeaxanthin-antheraxanthin-violaxanthin (VAZ pathway) and diadinoxanthine-diatoxanthin (DD-DT pathway) (Figure 1) are reversible and serve to dissipate cellular energy upon exposure to high light. This mechanism of photoproduction is called the xanthophyll cycle [101; 102].