The Genera Rhodothermus, Thermonema, Hymenobacter and Salinibacter

Five brightly red-pigmented, motile, rod-shaped, extremely halophilic bacteria were isolated from saltern crystallizer ponds in Alicante (two strains) and Mallorca (three strains), Spain. They grew optimally at salt concentrations between 20 and 30% and did not grow below 15% salts. Thus, these isolates are among the most halophilic organisms known within the domain Bacteria. The temperature optimum was 37-47 degrees C. A single, yet to be identified pigment was present, with an absorption maximum at 482 nm and a shoulder at 506-510 nm. The G+C content of the DNA was 66.3-67.7 mol% and, together, they formed a homogeneous genomic group with DNA-DNA similarities above 70%. The 16S rRNA gene sequences were almost identical to sequences recovered earlier from the saltern biomass by amplification of bacterial small-subunit rRNA genes from DNA extracted from the environment. This phylotype, earlier described as 'Candidatus Salinibacter', was shown by fluorescence in situ hybridization to contribute between 5 and 25% of the prokaryote community of the saltern crystallizers. We have therefore succeeded in isolating a bacterium from the natural environment that, although being a major component of the community, was previously known by its phylotype only. Isolation of the organism now allows formal description of a novel genus and species, for which we propose the name Salinibacter ruber gen. nov., sp. nov. The type strain is strain M31T (= DSM 13855T = CECT 5946T).

It is generally assumed that hypersaline environments with sodium chloride concentrations close to saturation are dominated by halophilic members of the domain Archaea, while Bacteria are not considered to be relevant in this kind of environment. Here, we report the high abundance and growth of a new group of hitherto-uncultured Bacteria in crystallizer ponds (salinity, from 30 to 37%) from multipond solar salterns. In the present study, these Bacteria constituted from 5 to 25% of the total prokaryotic community and were affiliated with the Cytophaga-Flavobacterium-Bacteroides phylum. Growth was demonstrated in saturated NaCl. A provisional classification of this new bacterial group as "Candidatus Salinibacter gen. nov." is proposed. The perception that Archaea are the only ecologically relevant prokaryotes in hypersaline aquatic environments should be revised.

The accumulation of organic solutes is a prerequisite for osmotic adjustment of all microorganisms. Thermophilic and hyperthermophilic organisms generally accumulate very unusual compatible solutes namely, di-myo-inositol-phosphate, di-mannosyl-di-myo-inositol-phosphate, di-glycerol-phosphate, mannosylglycerate and mannosylglyceramide, which have not been identified in bacteria or archaea that grow at low and moderate temperatures. There is also a growing awareness that some of these compatible solutes may have a role in the protection of cell components against thermal denaturation. Mannosylglycerate and di-glycerol-phosphate have been shown to protect enzymes and proteins from thermal denaturation in vitro as well, or better, than compatible solutes from mesophiles. The pathways leading to the synthesis of some of these compatible solutes from thermophiles and hyperthermophiles have been elucidated. However, large numbers of questions remain unanswered. Fundamental and applied interest in compatible -solutes and osmotic adjustment in these organisms, drives research that, will, in the near future, allow us to understand the role of compatible solutes in osmotic protection and thermoprotection of some of the most fascinating organisms known on Earth.