True marine and halophilic anoxygenic phototrophic bacteria

Anoxygenic phototrophic bacteria are widely distributed in marine sediments and shallow waters of the coastal zone, where they often form intensely colored mass developments. The phototrophic bacteria have adapted to the whole spectrum of salt concentrations, from freshwater to saturated brines, and it is apparent that individual species have adapted well to particular habitats and mineral salts compositions, both qualitatively and quantitatively. This adaptation is reflected not only in the demand for defined ranges of salt concentrations, but also in the phylogenetic relationships of these bacteria, as established by 16S rDNA sequences. Major phylogenetic branches of purple sulfur bacteria are represented by: (1) marine and extremely halophilic Ectothiorhodospiraceae, (2) truly marine and halophilic Chromatiaceae and (3) freshwater Chromatiaceae, some of which are tolerant to low salt concentrations and are successful competitors in brackish and marine habitats. Quite similarly, salt-dependent green sulfur bacteria form distinct phylogenetic lines. In addition, also among the phototrophic alpha-Proteobacteria (purple nonsulfur bacteria), distinct phylogenetic lines of salt-dependent species are recognized. Available data give rise to the assumption that salt concentrations of natural habitats are an important selective factor that determines the development of a selected range of phototrophic bacteria in an exclusive way. As a consequence, the salt responses of these bacteria are reflected in their phylogenetic relationships.