ABSTRACT
Carotenoids are pigments varying in color from yellow to red which occur in cells of all auto- and heterotrophic plants. Carotenoids form a subgroup of polyene pigments generally containing 40 carbon atoms in the molecule. The biosynthesis of carotenoids in plants begins in a pathway common to all terpenes; the first specific carotenoid precursor in the pathway is phytoene. The characteristic acyclic tetraterpene of carotenoids is formed through the dimerization of 20-C molecules of geranyl geranyl pyrophosphate. In addition, cyclic carotenoids may be formed from 40-C acyclic precursors of neurosporene or lycopene. The largest group found are the xanthophylls comprising 33 carotenoids; the most common forms are the hydroxyls, the epoxides, and the ketones. The presence of various carotenoids, total carotenoid content, and the predominant carotenoids in the same species of lichen from various geographical regions may considerably. In all species growing in shaded places, carotenoids play an essential role as antennae which absorb light rays, particularly in the cyanobionts.
