The genus Roseomonas is a significant group of bacteria which is invariably of great clinical and ecological importance. Previous studies have shown that the genus Roseomonas is polyphyletic in nature. Our present study focused on generating a lucid understanding of the phylogenetic framework for the re-evaluation and reclassification of the genus Roseomonas. Phylogenetic studies based on the 16S rRNA gene and 92 concatenated genes suggested that the genus is heterogeneous, forming seven major groups. Existing Roseomonas species were subjected to an array of genomic, phenotypic, and chemotaxonomic analyses in order to resolve the heterogeneity. Genomic similarity indices (dDDH and ANI) indicated that the members were well-defined at the species level. The Percentage of Conserved Proteins (POCP) and the average Amino Acid Identity (AAI) values between the groups of the genus Roseomonas and other interspersing members of the family Acetobacteraceae were below 65 and 70%, respectively. The pan-genome evaluation depicted that the pan-genome was an open type and the members shared 958 core genes. This claim of reclassification was equally supported by the phenotypic and chemotaxonomic differences between the groups. Thus, in this study, we propose to re-evaluate and reclassify the genus Roseomonas and propose six novel genera as Pararoseomonas gen. nov., Falsiroseomonas gen. nov., Paeniroseomonas gen. nov., Plastoroseomonas gen. nov., Neoroseomonas gen. nov., and Pseudoroseomonas gen. nov.

The genus Rhodobacter is taxonomically well studied, and some members are model organisms. However, this genus is comprised of a heterogeneous group of members. 16S rRNA gene-based phylogeny of the genus Rhodobacter indicates a motley assemblage of anoxygenic phototrophic bacteria (genus Rhodobacter) with interspersing members of other genera (chemotrophs) making the genus polyphyletic. Taxogenomics was performed to resolve the taxonomic conflicts of the genus Rhodobacter using twelve type strains. The phylogenomic analysis showed that Rhodobacter spp. can be grouped into four monophyletic clusters with interspersing chemotrophs. Genomic indices (ANI and dDDH) confirmed that all the current species are well defined, except Rhodobacter megalophilus. The average amino acid identity values between the monophyletic clusters of Rhodobacter members, as well as with the chemotrophic genera, are less than 80% whereas the percentage of conserved proteins values were below 70%, which has been observed among several genera related to Rhodobacter. The pan-genome analysis has shown that there are only 1239 core genes shared between the 12 species of the genus Rhodobacter. The polyphasic taxonomic analysis supports the phylogenomic and genomic studies in distinguishing the four Rhodobacter clusters. Each cluster is comprised of one to seven species according to the current Rhodobacter taxonomy. Therefore, to address this taxonomic discrepancy we propose to reclassify the members of the genus Rhodobacter into three new genera, Luteovulum gen. nov., Phaeovulum gen. nov. and Fuscovulum gen. nov., and provide an emended description of the genus Rhodobacter sensu stricto. Also, we propose reclassification of Rhodobacter megalophilus as a sub-species of Rhodobacter sphaeroides.

Two bacterial strains (JA744^T and JA745) were isolated from dry soil samples collected from solar salterns at Humma, Odisha, India. Both strains were Gram-stain-negative, catalase- and oxidase-positive, motile rods. Major fatty acids in both strains included C_18 : 1ω7c, C_18 : 0 and C_16 : 0, while minor amounts of C_10 : 0 3-OH, C_12 : 0, C_12 : 0 3-OH, C_14 : 0 and C_16 : 0 were also present. Diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylcholine, phosphatidylglycerol, an unidentified glycolipid, five unidentified lipids, an unidentified aminolipid and an unidentified phospholipid made up the polar lipids of both strains. Both strains had bacteriohopane derivatives (BHD1,2) and diploptene as major hopanoids. Mean genomic DNA G+C content was 75±1 mol% and the two strains were closely related (mean DNA–DNA hybridization >90 %). Phylogenetic analysis based on the 16S rRNA gene sequence showed that the two strains clustered with species of the genus Rhodobacter belonging to the family Rhodobacteraceae of the class Alphaproteobacteria . The highest sequence similarity was observed with Rhodobacter sphaeroides ATH2.4.1^T (96 %) and other members of the genera Rhodobacter and Pseudorhodobacter (<96 %). However, the two strains were positioned distinctly outside the group formed by the other genera of the family Rhodobacteraceae . Distinct morphological, physiological and genotypic differences from previously described taxa support the classification of these isolates as representatives of a novel species in a new genus, for which the name Falsirhodobacter halotolerans gen. nov., sp. nov. is proposed. The type strain of Falsirhodobacter halotolerans is JA744^T ( = KCTC 32158^T = NBRC 108897^T).