Halomonas neptunia sp. nov., Halomonas sulfidaeris sp. nov., Halomonas axialensis sp. nov. and Halomonas hydrothermalis sp. nov.: halophilic bacteria isolated from deep-sea hydrothermal-vent environments

Abstract

To assess the physiological and phylogenetic diversity of culturable halophilic bacteria in deep-sea hydrothermal-vent environments, six isolates obtained from low-temperature hydrothermal fluids, sulfide rock and hydrothermal plumes in North and South Pacific Ocean vent fields located at 1530–2580 m depth were fully characterized. Three strains were isolated on media that contained oligotrophic concentrations of organic carbon (0·002 % yeast extract). Sequencing of the 16S rRNA gene indicated that all strains belonged to the genus Halomonas in the γ-subclass of the Proteobacteria. Consistent with previously described species, the novel strains were slightly to moderately halophilic and grew in media containing up to 22–27 % total salts. The isolates grew at temperatures as low as −1 to 2 °C and had temperature optima of 30 or 20–35 °C. Both the minimum and optimum temperatures for growth were similar to those of Antarctic and sea-ice Halomonas species and lower than typically observed for the genus as a whole. Phenotypic tests revealed that the isolates were physiologically versatile and tended to have more traits in common with each other than with closely related Halomonas species, presumably a reflection of their common deep-sea, hydrothermal-vent habitat of origin. The G+C content of the DNA for all strains was 56·0–57·6 mol%, and DNA–DNA hybridization experiments revealed that four strains (Eplume1T, Esulfide1T, Althf1T and Slthf2T) represented novel species and that two strains (Eplume2 and Slthf1) were related to Halomonas meridiana. The proposed new species names are Halomonas neptunia (type strain Eplume1T=ATCC BAA-805T=CECT 5815T=DSM 15720T), Halomonas sulfidaeris (type strain Esulfide1T=ATCC BAA-803T=CECT 5817T=DSM 15722T), Halomonas axialensis (type strain Althf1T=ATCC BAA-802T=CECT 5812T=DSM 15723T) and Halomonas hydrothermalis (type strain Slthf2T=ATCC BAA-800T=CECT 5814T=DSM 15725T).