Reclassification of species of the spiral-shaped phototrophic purple non-sulfur bacteria of the {alpha}-Proteobacteria: description of the new genera Phaeospirillum gen. nov., Rhodovibrio gen. nov., Rhodothalassium gen. nov. and Roseospira gen. nov. as well as transfer of Rhodospirillum fulvum to Phaeospirillum fulvum comb. nov., of Rhodospirillum molischianum to Phaeospirillum molischianum comb. nov., of Rhodospirillum salinarum to Rhodovibrio salinarum comb, nov., of Rhodospirillum sodomense to Rhodovibrio sodomensis comb. nov., of Rhodospirillum salexigens to Rhodothalassium salexigens comb. nov. and of Rhodospirillum mediosalinum to Roseospira mediosalina comb. nov.

This study reclassifies spiral-shaped phototrophic purple non-sulfur bacteria previously grouped in the genus Rhodospirillum based on 16S rDNA sequence analysis. The authors determined the 16S rDNA sequence of Rhodospirillum mediosalinum and compared it with other Rhodospirillum species, revealing five phylogenetically distinct groups with less than 91% sequence similarity to each other. These genetic differences correlated with phenotypic variations in quinone composition, fatty acid profiles, and salt requirements. The researchers propose four new genera: Phaeospirillum (containing P. fulvum and P. molischianum), Rhodovibrio (R. salinarum and R. sodomensis), Rhodothalassium (R. salexigens), and Roseospira (R. mediosalina). The original genus Rhodospirillum is retained for R. rubrum and R. photometricum only. The taxonomic reorganization reflects both molecular phylogeny and physiological characteristics, with salt requirement emerging as a genus-specific property that distinguishes freshwater from halophilic lineages within these bacteria.

Key findings

• 16S rDNA sequence analysis revealed five phylogenetically distinct groups among Rhodospirillum species with less than 91% similarity between groups, necessitating reclassification into separate genera.
• Four new genera were established: Phaeospirillum, Rhodovibrio, Rhodothalassium, and Roseospira, with six species transferred from Rhodospirillum.
• Genetic groupings correlated strongly with phenotypic differences in major quinone components (ubiquinones, menaquinones, rhodoquinones), fatty acid composition, and salt requirements for growth.
• Salt dependence was identified as a genus-specific diagnostic property, distinguishing halophilic bacteria from freshwater species across these genera.
• Only Rhodospirillum rubrum and R. photometricum retained the original genus designation based on phylogenetic analysis and phenotypic characteristics.