Diversity of anaerobic anoxygenic phototrophic purple bacteria

Abstract

The anaerobic phototrophic purple bacteria are a phylogenetically highly diverse group of bacteria with the common physiological property of performing anoxygenic photosynthesis under anaerobic conditions. They are representatives of the Alpha-, Beta-, and Gammaproteobacteria. More than 160 known species are classified into 57 genera, 12 families, and 7 orders of the Proteobacteria. A short historical review on the systematic treatment of the phototrophic purple bacteria and the actual state is presented. The phylogenetic relationship of the cultured phototrophic purple bacteria based upon 16S rRNA gene sequences of the type strains is shown in a comprehensive phylogenetic tree. Correlation and differences between taxonomic treatment and phylogenetic relatedness are discussed. As the 16S rRNA gene is only of limited value for biodiversity studies of functional groups of bacteria, the application of functional genes for these purposes gains importance and genes coding for bacteriochlorophyll synthesis (bchY), and reaction center proteins (pufL and pufM) have been applied for biodiversity studies of phototrophic purple bacteria. The correlation of phylogenetic relationships based on pufLM gene sequences with that of 16S rRNA gene sequences, the appropriate information content in the pufLM sequences (>1400 nt), and a database of pufLM sequences from many of the cultured purple bacteria make these genes a preferred target to study environmental communities of phototrophic purple bacteria. Studies on the diversity of phototrophic purple bacteria in three representative habitats are discussed: an exceptionally well-developed microbial mat in a salt marsh from which a number of new phototrophic purple bacteria have been isolated (Sippewissett Salt Marsh, MA), the chemocline of a meromictic freshwater lake from which new species have been isolated and in which genetic diversity studies have been performed (Lake Cadagno, Switzerland), and a coastal lagoon (Baltic Sea lagoon at Stein, Germany) in which the diversity of phototrophic purple bacteria and the impact of changes in temperature and salinity on the community composition was studied using almost complete sequences of the pufLM genes.