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Classification of purple bacteria taxonomy
Purple bacteria, including the aerobic bacteria belong to phylum proteobacteria; this phylum was established by Carl Woese in 1987 calling it “purple bacteria and their relatives” even if this is not appropriate because most of them are not purple or photosynthetic. Phylum Proteobacteria is divided into six classes:Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Deltaproteobacteria, Epsilonproteobacteria and Zetaproteobcateria. Purple bacteria are distributed between 3 classes:Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria. each caracterized by a photosynthetic phenotype.

Alpha subdivision contanis different photosynthetic species (as Rhodospirillum, Rhodopseudomonas and Rhodomicrobium) but include also some non-photosyntetic ones of genera with nitrogen metabolism ( Rhizobium, Nitrobacter) whereas in betaproteobacteria subdivision there are few photosynthetic species. Gammaproteobacteria class is divided into 3 subgroups : gamma-1, gamma-2, gamma-3. In gamma-1 subgroup there are the purple photosynthetic bacteria that produce molecular sulfur (Chromatiaceae group and Ectothiorhodospiraceae group) and also the non-photosyntetic species ( as Nitrosococcus oceani )

Purple sulfur bacteria and purple nunsulfur bacteria were distinguished on physiological factors of their tolerance and utilization of sulfide: was considered that purple sulfur bacteria tolerate millimolar levels of sulfide and oxidized sulfide to sulfur globules stored intracellulary while purple nonsulfur bacteria species did neither. This kind of classification was not absoluted. It’s was refuted with classic chemostat experiments by Hansen and Van Gemerden (1972) that demonstrate the growing of many purple nonsulfur bacteria species, at low levels of sulfide (0.5mM) and in so doing, oxidize sulfide to S0, S4O62–, or SO42–. The important distinction that remains from theese two different metabolisms is that: any S0 formed by purple nonsulfur bacteria is not stored intracellularly but is deposited outside the cell. (even if there are exception for this as Ectothiorhodospiraceae). So if grown on sulfide it is easy to differentiate purple sulfur bacteria from purple non sulfur bacteria because the microscopically globules of S0 are formed.