Carotenoids are among the natural products with the highest diversity. To date more than 700 different naturally occurring carotenoids have been described (Britton et al., 2004), and they are virtually ubiquitous in living organisms. Carotenoids belong to the compound class of isoprenoids, and the majority of carotenoids are tetraterpenoids with a C40 skeleton as the basic molecular structure. Formally, they can be divided into the carotenes, which are pure hydrocarbons, and the xanthophylls, which are derived from carotenes by introduction of oxygen functions. The ability of de novo biosynthesis of carotenoids is not limited to land plants and algae, but is frequently encountered among prokaryotes (both eubacteria and archaebacteria) and fungi (Britton, 1998). To meet their metabolic demands, animals rely on food-borne uptake of carotenoids which then, however, can be further metabolized. An important example is β-carotene (provitamin A; trivial names for carotenes are used in this chapter – see Data sheets for alternative names such as β,β-carotene in this case) as a precursor of the visual pigment retinal in mammals and animals in general (Goodwin, 1984; von Lintig et al., 2005) or – together with its hydroxylated derivatives – as precursors of ketocarotenoids used as colourants by many crustaceans, various carp species and birds like flamingos or finches (Goodwin, 1984; McGraw et al., 2006). This chapter summarizes our current view on the biosynthesis of carotenoids, termed carotenogenesis, in land plants, cyanobacteria and algae with a focus on recent advances in the genetics of carotenoid biosynthesis. The scope of this review will be limited to oxygenic phototrophs; for details on the carotenogenesis of anoxygenic phototrophic bacteria, readers are referred to an excellent recent review by Maresca, Graham and Bryant (Maresca et al., 2008).
When the chapter on carotenoids in the volume by Jeffrey et al. (1997) was written, only the enzymes of the cytosolic mevalonic acid pathway of isoprene formation and those catalysing the early steps of carotenoid biosynthesis up to the carotenes were known. During the last decade, most of the genes involved in carotenogenesis in cyanobacteria and in land plants have been identified, and a new pathway of isoprene formation that is operative in many bacteria and in plastids has been discovered. Most of our current knowledge about the biosynthesis of carotenoids in oxygenic phototrophs stems from work on seed plants and cyanobacteria, and to some extent on green algae. Almost no experimental data are available from other algal groups, but increasingly more whole-genome data are generated enabling homology-based identification of potential carotenogenic genes in these algae. As will be discussed, the emerging picture is rather complex with a significant number of reactions being catalysed by multiple enzymes which are often phylogenetically unrelated. Other enzymes share the same ancestor, but yet catalyse different reactions. In the following, the known enzymatic steps of carotenogenesis in oxygenic phototrophs will be summarized, with special reference to carotenoid biosynthesis in algae and cyanobacteria.