Anoxygenic phototrophic purple bacteria are a major group of photosynthetic microorganisms widely distributed in nature, primarily
in aquatic habitats. Nearly 50 genera of these organisms are known and some have become prime model systems for the experimental
dissection of photosynthesis. Purple sulfur bacteria differ from purple nonsulfur bacteria on both metabolic and phylogenetic
grounds, but species of the two major groups often coexist in illuminated anoxic habitats in nature. Purple sulfur bacteria
are strong photoautotrophs and capable of limited photoheterotrophy, but they are poorly equipped for metabolism and growth
in the dark. By contrast, purple nonsulfur bacteria, nature’s preeminent photoheterotrophs, are capable of photoautotrophy,
and possess diverse capacities for dark metabolism and growth. Several purple bacteria inhabit extreme environments, including
extremes of temperature, pH, and salinity. Collectively, purple bacteria are important phototrophs because they (1) consume
a toxic substance, H₂S, and contribute organic matter to anoxic environments by their autotrophic capacities; (2) consume organic compounds, primarily
non-fermentable organic compounds, in their roles as photoheterotrophs; and (3) offer scientists in the photosynthesis community
a smörgasbord of molecular diversity for the study of photosynthesis.

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.