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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

¹ Laboratoire de Microbiologie, IMEP, Faculté des Sciences et Techniques de Saint Jérôme, 13397 Marseille cedex 20, France
² Laboratoire de Biochimie et Biophysique des Systèmes Intégrés, DBMS/BBSI, CEA Grenoble, 38054 Grenoble, France
³ Division of Environmental and Applied Biology, Biological Sciences Institute, University of Dundee, Dundee DD1 4HN, UK
⁴ Laboratoire d'Ecologie Moléculaire-Microbiologie, IBEAS, BP 1155, Université de Pau, F 64013 Pau cedex, France

Correspondence
Pierre Caumette
pierre.caumette{at}univ-pau.fr

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 3301^T, 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 °C. The DNA base composition of strains SG 3301^T and SG 3304 were 74·5 and 74·1 mol% G+C, respectively. According to the 16S rDNA sequences, strains SG 3301^T 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 3301^T as the type strain (=DSM 15116^T).

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