Distribution and Biosynthesis of Carotenoids

Abstract

Purple bacteria including aerobic photosynthetic bacteria belong to the Proteobacteria, and 75 genera including around 160 species have been described. These bacteria produce around 100 different carotenoids, which are essential for photoprotection and light-harvesting. This chapter summarizes the distribution and biosynthesis of carotenoids in all of the purple bacteria described so far. All of the carotenogenesis genes from Rhodobacter capsulatus, Rhodobacter sphaeroides and Rubrivivax gelatinosus, and some genes from other purple bacteria have been functionally confirmed, and the characteristics of their products have been investigated. When one enzyme of the typical spirilloxanthin pathway is lacking or is present with reduced activity, the carotenoid composition of the bacterium will be expected to change; indeed, the variation of the spirilloxanthin pathways can be explained by this idea. Based on these new findings, two main pathways within purple bacteria have been proposed; the spirilloxanthin pathway (normal spirilloxanthin, unusual spirilloxanthin, spheroidene, and carotenal pathways) and the okenone pathway (okenone and R.g.-keto carotenoid pathways). In addition, carotenoid glucosides and carotenoid glucoside fatty acid esters have also been found in some species. Purple bacteria classified as Alphaproteobacteria and Betaproteobacteria have the spirilloxanthin pathway, while those in the Gammaproteobacteria have either the spirilloxanthin or the okenone pathway depending on genus or species. The aerobic photosynthetic bacteria described so far are classified as Alphaproteobacteria and Betaproteobacteria, and most species have the spirilloxanthin pathway. Furthermore most of these species also have unusual carotenoids including ‘non-photosynthetic’ carotenoids, such as carotenoid sulfates and carotenoic acids, which seem to have no photosynthetic functions.