Phylogenetic relationships among budding purple bacteria of the genus Rhodopseudomonas

Phylogenetic relationships within a group of budding purple nonsulfur bacteria of the genus Rhodopseudomonas were studied by DNA-DNA hybridization. Most of the strains were new isolates from natural environments with a wide ränge of physicochemical conditions.A total of 32 strains were analyzed, includ- ing the type strains of Rhodopseudomonas palustris, Rps. rutila, Rps. marina, and Rps. acidophila. Nine phylo- genetically remote clusters were revealed, showing an average DNA-DNA horaology of 15%. One of the Clusters contained the type strain of Rps. marina, isolated front marine habitats, and two new isolates from hot ffeshwater springs. These new isolates tumed out to be facultative halophiles with a salinity Optimum of 1-3% and were assigned to Rps. marina on the basis of their phenotypic characteristics and DNA-DNA homology of 60% with the type strain of this species. The type strain of Rps. acidophila was not specifically related to any other strain used during this investigation. The remaining 26 isolates and the type strains of Rps. palustris and Rps. rutila were very similar phenotypically, but formed seven phylogenetically remote clusters including 13 genospecies; none of the new isolates were closely related to the type strains of Rps. palustris and Rps. rutila, which also proved to be phylogenetically distinct. Thus, the Rps. palustris-like bacteria, which are widespread in nature and comprise phenotypically a homogeneous group, appear to be phylogenetically diverse. The prospects for developing a Classification system for these budding purple nonsulfur bacteria that would be in agreement with their phylogenetic relationships are discussed. Possible correlation between the phylogenetic relationships of these bacteria and the ecological peculiarities of their natural habitats is also considered