Phylogeny of the family Halomonadaceae based on 23S and 16S rDNA sequence analyses

In this study, we have evaluated the phylogenetic status of the family Halomonadaceae, which consists of the genera Halomonas, Chromohalobacter and Zymobacter, by comparative 23S and 16S rDNA analyses. The genus Halomonas illustrates very well a situation that occurs often in bacterial taxonomy. The use of phylogenetic tools has permitted the grouping of several genera and species believed to be unrelated according to conventional taxonomic techniques. In addition, the number of species of the genus Halomonas has increased as a consequence of new descriptions, particularly during the last few years, but their features are too heterogeneous to justify their placement in the same genus and, therefore, a re-evaluation seems necessary. We have determined the complete sequences (about 2900 bases) of the 23S rDNA of 18 species of the genera Halomonas and Chromohalobacter and resequenced the complete 16S rDNA sequences of seven species of Halomonas. The results of our analysis show that two phylogenetic groups (respectively containing five and seven species) can be distinguished within the genus Halomonas. Six other species cannot be assigned to either of the above-mentioned groups. Furthermore, Halomonas marina forms a separate branch at a deeper level than the other species of the genus Halomonas, which suggests that it should be ascribed to a separate genus. The genus Chromohalobacter forms a monophyletic group constituted by Chromohalobacter marismortui, the recently reclassified species Chromohalobacter canadensis and Chromohalobacter israelensis and the recently proposed species Chromohalobacter salexigens. Finally, we propose to include the genus Carnimonas, with its single species Carnimonas nigrificans, in the family Halomonadaceae.