Isolation and characterization of spirilloid purple phototrophic bacteria forming red layers in microbial mats of Mediterranean salterns: description of Halorhodospira neutriphila sp. nov. and emendation of the genus Halorhodospira.

Microbial mats developing in the hypersaline lagoons of a commercial saltern in the Salin-de-Giraud (Rhône delta) were found to contain a red layer fully dominated by spirilloid phototrophic purple bacteria underlying a cyanobacterial layer. From this layer four strains of spirilloid purple bacteria were isolated, all of which were extremely halophilic. All strains were isolated by using the same medium under halophilic photolithoheterotrophic conditions. One of them, strain SG 3105 was a purple non-sulfur bacterial strain closely related to Rhodovibrio sodomensis with a 16S rDNA sequence similarity of 98.8%. The three other isolated strains, SG 3301T, SG 3302 and SG 3304, were purple sulfur bacteria and were found to be very similar. The cells were motile by a polar tuft of flagella. Photosynthetic intracytoplasmic membranes of the lamellar stack type contained BChl a and spirilloxanthin as the major carotenoid. Phototrophic growth with sulfide as electron donor was poor; globules of elemental sulfur were present outside the cells. In the presence of sulfide and CO2 good growth occurred with organic substrates. Optimum growth occurred in the presence of 9-12% (w/v) NaCl at neutral pH (optimal pH 6.8-7) and at 30-35 degrees C. The DNA base composition of strains SG 3301T and SG 3304 were 74.5 and 74.1 mol% G + C, respectively. According to the 16S rDNA sequences, strains SG 3301T and SG 3304 belonged to the genus Halorhodospira, but they were sufficiently separated morphologically, physiologically and genetically from other recognized Halorhodospira species to be described as a new species of the genus. They are, therefore, described as Halorhodospira neutriphila sp. nov. with strain SG 3301T as the type strain (=DSM 15116T).

Characterization of three spiral-shaped purple nonsulfur bacteria isolated from coastal lagoon sediments, saline sulfur springs, and microbial mats: emended description of the genus Roseospira and description of Roseospira marina sp. nov., Roseospira navarrensis sp. nov., and Roseospira thiosulfatophila sp. nov.

Three new spirilloid phototrophic purple nonsulfur bacteria were isolated in pure culture from three different environments: strain CE2105 from a brackish lagoon in the Arcachon Bay (Atlantic coast, France), strain SE3104 from a saline sulfur spring in the Pyrenees (Navarra, Spain), and strain AT2115 a microbial mat (Tetiaroa Atoll, Society Islands). Single cells of the three strains were spiral-shaped and highly motile. Their intracellular photosynthetic membranes were of the vesicular type. Bacteriochlorophyll a and carotenoids of the normal spirilloxanthin series were present as photosynthetic pigments. Optimal growth occurred under photoheterotrophic conditions and in the presence of 0.5-4% w/v NaCl. These features are similar to those described for Roseospira mediosalina. Comparative sequence analysis of their 16S rRNA genes placed these strains within the alpha-subclass of Proteobacteria, in a cluster together with Roseospira mediosalina and Rhodospira trueperi. They form a closely related group of slightly to moderately halophilic spiral-shaped purple nonsulfur bacteria.However, the three new isolates exhibited some differences in their physiology and genetic characteristics. Consequently, we propose that they are members of three new species within the genus Roseospira, Roseospira marina sp. nov., Roseospira navarrensis sp. nov., and Roseospira thiosulfatophila sp. nov., with strains CE2105, SE3104, and AT2115 as the type strains, respectively. As a consequence, an emended description of the genus Roseospira is also given.