Anoxygenic Phototrophic Purple Bacteria

Abstract

The anoxygenic phototrophic purple bacteria represent an assemblage of predominantly aquatic bacteria that are able to grow under anoxic conditions by photosynthesis, without oxygen production. The major pigments that help in photosynthesis are bacteriochlorophyll a or b and various carotenoids of the spirilloxanthin, rhodopinal, spheroidene, and okenone series, which originate from the cytoplasmic membrane. These bacteria are unable to use water as an electron donor and instead need more reduced compounds. Purple nonsulfur bacteria belong to Alphaproteobacteria and the Betaproteobacteria, and purple sulfur bacteria belong to the Gammaproteobacteria. The overwhelming evidence of molecular data led to major taxonomic reclassifications of these bacteria. Based on 16S rDNA sequence analysis, the phototrophic Alphaproteobacteria in the order Rhodospirillales are phylogenetically distinct from other groups of phototrophic Alphaproteobacteria, though they are closely related to several purely chemotrophic representatives of this group. These members have characteristic phospholipid and fatty acid composition with C18:1 as the dominant fatty acid and either C16:1 and C16:0, C16:0 and C18:0, or just C16:0 as additional major components. Major differentiating properties of the genera and species of Rhodopseudomonas, Rhodobium, Rhodoplanes, Blastochloris, Rhodoblastus, and Rhodomicrobium are also described. In phototrophic Betaproteobacteria, sulfate can be assimilated as the sole sulfur source and is reduced with adenosine-5′-phosphosulfate (APS) as an intermediate. NADH is used as a cosubstrate in the glutamine synthetase/glutamate synthase reactions, and high-potential iron–sulfur protein is present. These species were included in the Rhodospirillaceae together with the phototrophic Alphaproteobacteria. Phototrophic purple sulfur bacteria that are able to perform photosynthesis under anoxic conditions without oxygen production, that preferentially use reduced sulfur compounds as photosynthetic electron donors, and that grow photolithoautotrophically are Gammaproteobacteria. These species are classified with the Chromatiaceae and Ectothiorhodospiraceae of the Chromatiales. Keywords: phototroph; Chromatiaceae; Ectothiorhodospiraceae; purple nonsulfur bacteria