Rubribacterium polymorphum gen. nov., sp. nov., a novel alkaliphilic nonsulfur purple bacterium from an Eastern Siberian soda lake

An alkaliphilic nonsulfur purple bacterial (NPB) strain “Green” was isolated from sediments of the littoral zone of the soda lake (mineralization 22 g/l, pH 9.5) in the Barguzin River valley (Eastern Siberia). The cells of the new isolate are ovoid or polymorphic at latter stages. The photosynthetic membrane structures are of vesicular type. Bacteriochlorophyll a and carotenoids of both spheroidene and spirilloxanthin type are the photosynthetic pigments. Two light-harvesting systems (LH1 and LH2) are present. The new isolate is a photoheterotroph and a facultative aerobe. It grows well in the dark on organic substrates; anaerobic phototrophic growth is poor. The isolate is alkaliphilic with pH optimum of 8.5–9.5. The most abundant cell growth occurred at 5–40 g/l NaCl (optimum at 10 g/l) and 30 °C. The DNA G+C base content was 69.9 mol %. Analysis of 16S rRNA gene sequences revealed a 10% difference with the most closely related NPB (Rhodobacter species). Rubrimonas cliftoensis, a bacteriochlorophyll a-containing bacterium, is the closest relative (93.3% similarity). It is proposed that strain “Green” should be placed in the new genus and new species Rubribacterium polymorphum gen. nov., sp. nov. GenBank accession number: 16S rRNA-EU857676.