Systematics of Anoxygenic Phototrophic Bacteria

Abstract

Many of the anoxygenic phototrophic bacteria, in particular green sulfur bacteria and purple sulfur bacteria are actively involved in the dissimilatory sulfur cycle by oxidizing reduced sulfur compounds. An introduction to the current state of the systematics of anoxygenic phototrophic bacteria is given here. With the introduction of 16S rDNA sequences, the consideration of genetic relatedness of these bacteria and a great deal of chemotaxonomic properties, the systematic treatment of many of these bacteria has changed over the past decades. Many species and strains have been reclassified and new higher taxa were established that harbour the phototrophic genera. Four major phylogenetic groups that have significant phenotypic characteristics can be distinguished: (1) the Heliobacteria (Heliobacteriaceae), which are Gram-positive bacteria, (2) the filamentous and gliding green bacteria (Chloroflexaceae), and (3) the green sulfur bacteria (Chlorobiaceae), which each form separate phylogenetic lines within the eubacteria, and (4) the purple sulfur and nonsulfur bacteria (various taxa among the Alpha-, Beta- and Gammaproteobacteria). This chapter concentrates on the following groups of which major properties and representative genera and species are treated: The purple sulfur bacteria are Gammaproteobacteria and treated as Chromatiales (Ectothiorhodospiraceae and Chromatiaceae families). The purple nonsulfur bacteria are Betaproteobacteria (Comamonadaceae of the Burkholdriales and Rhodocyclaceae of Rhodocyeclales) and Alphaproteobacteria ( Rhodospirillaceae, Acetobacteraceae, Rhodobacteraceae, Bradyrhizobiaceae, Hyphomicrobiaceae, Rhodobiaceae) and are closely related to non-phototrophic purely chemotrophic relatives and to so-called aerobic bacteriochlorophyll-containg bacteria. The green sulfur bacteria form a closely related cluster of genera united in a single family the Chlorobiaceae, which is according to its isolated position within the phylogenetic tree of bacteria recognised as a separate phylum, the Chlorobi. Because of the difficulties related to taxonomic treatment and phylogenetic groupings, a list of strains is given indicating old and new taxonomic names of species and genus.