Carotenoids are synthesized in nature by plants and many microorganisms. In addition to very few bacterial carotenoids with 30, 45, or 50 carbon atoms, C40-carotenoids represent the majority of the more than 600 known structures. Two groups have been singled out as the most important: the carotenes which are composed of only carbon and hydrogen; and the xanthophylls, which are oxygenated derivatives (Frengova & Beshkova, 2009). In the
later, oxygen can be present as OH groups, or as oxy-groups or in a combination of both (as in astaxanthin). Hydroxy groups at the ionone ring may be glycosylated or carry a glycoside fatty acid ester moiety. Furthermore, carotenoids with aromatic rings or acyclic structures with different polyene chains and typically 1-methoxy groups can also be found. Typical fungal carotenoids possess 4-keto groups, may be monocyclic, or possess 13 conjugated double bonds (Britton et al., 1998).

All carotenoids are derived from the isoprenoid or terpenoid pathway. Carotenoids biosynthesis pathway commonly involves three steps: (i) formation of isopentenyl pyrophosphate (IPP), (ii) formation of phytoene and (iii) cyclization and other reactions of lycopene (Armstrong & Hearst, 1996). Before polyprenyl formation begins, one molecule of IPP must be isomerized to DMAPP. Condensation of one molecule of dimethylallyl diphosphate (DMADP) and three molecules of isopentenyl diphosphate (IDP) produces the diterpene geranylgeranyl diphosphate (GGDP) that forms one half of all C40 carotenoids. The head to head condensation of two GGDP molecules results in the first colorless
carotenoid, phytoene. As Figure 9 shows, phytoene synthesis is the first committed step in C40-carotenoid biosynthesis (Britton et al., 1998; Sandmann, 2001). Subsequent desaturation reactions lengthen the conjugated double bond system to produce neurosporene or lycopene (Schmidt-Dannert, 2000).

Following desaturation, carotenoid biosynthesis branches into routes for acyclic and cyclic carotenoids. In phototrophic bacteria acyclic xanthophylls spheroidene or spheroidenone and spirilloxanthin, respectively are formed (Figure 9). Synthesis of cyclic carotenoids involves cyclization of one or both end groups of lycopene or neurosporene. Typically, P — rings are introduced, but formation of є-rings is common in higher plants and carotenoids with у-rings are found, for example, in certain fungi. Most cyclic carotenoids contain at least one oxygen function at one of the ring carbon atoms. Cyclic carotenoids with keto-groups at C4(C4′) and/or hydroxy groups at C3(C3′) (e. g. zeaxanthin, astaxanthin, echinenone and lutein) are widespread in microorganisms and plants (Schmidt-Dannert, 2000).