Carotenoids and Carotenogenesis in Anoxygenic Photosynthetic Bacteria

Abstract

More than 50 genera including about 130 species of anoxygenic photosynthetic bacteria have been described. These bacteria produce around 100 different carotenoids. In this chapter‚ the carotenoid compositions of all the photosynthetic bacteria so far described are summarized. All of the carotenogenesis genes from Rhodobacter‚ and some of them from other bacteria have been cloned‚ and the characteristics of their products have been investigated. Schmidt (1978) proposed four main pathways for carotenogenesis. In this chapter‚ five main pathways within anaerobic photosynthetic bacteria are now suggested based on these new findings: the spirilloxanthin pathway (normal spirilloxanthin‚ unusual spirilloxanthin‚ spheroidene‚ and carotenal pathways)‚ the okenone pathway (okenone‚ andR.g.-ketocarotenoidpathways)‚ the isorenieratene pathway (isorenieratene‚ and chlorobactene pathways)‚ the and pathway‚ and the diapocarotene pathway. In addition‚ carotenoid glucosides and carotenoid glucoside fatty acid esters have also been found in some species.The Rhodospirillaceae and Chromatiaceae have the spirilloxanthin or the okenone pathway depending on the genus or species. All of the Ectothiorhodospiraceae have the spirilloxanthin pathway. The isorenieratene‚ the and the diapocarotene pathways are found specifically in the Chlorobiaceae‚ Chloroflexaceae‚ and Heliobacteriaceae‚ respectively. Aerobic photosynthetic bacteria mostly have the spirilloxanthin pathway‚ further most of these species have unusual carotenoids including ‘non-photosynthetic’ carotenoids‚ such as carotenoid sulfates and carotenoic acids‚ which have no photosynthetic functions.