Phylogenomics and molecular marker-based analyses of the order Nevskiales: proposal for the creation of Steroidobacterales ord. nov. and Peristeroidobacter gen. nov.

The order Nevskiales, class Gammaproteobacteria, encompasses four families Algiphilaceae, Salinisphaeraceae, Nevskiaceae, and Steroidobacteraceae. The taxonomy of this order is structured from the inferences derived from the 16S rRNA gene and genome-based phylogenetic tree analyses. However, previous taxonomic studies of the order failed to incorporate most of the representatives from other established orders within the class Gammaproteobacteria. Other divergent members within the class Gammaproteobacteria were therefore overlooked. In this study, the taxonomy of the order Nevskiales was revisited using genome-based analyses with an expanded scope of outgroups representing the vast majority of the diversity within the class Gammaproteobacteria. Results from the phylogenetic analyses strongly supported the exclusion of the family Steroidobacteraceae from the order Nevskiales and further implied the assignment of the family into a novel order. In addition, the analyses also supported the reclassification of Steroidobacter gossypii, Steroidobacter soli, Steroidobacter agariperforans, and Steroidobacter agaridevorans into a novel genus. The identified conserved signature indels in 33 protein sequences further reinforced the new taxonomic assignments. Furthermore, the results of the average nucleotide identity and average amino acid identity analyses, together with the phenotypic and genomic characteristics among the members of the genus Steroidobacter also provided evidence supporting the reclassification of the four Steroidobacter species. Based on these results, the family Steroidobacteraceae is proposed to be assigned to a novel order Steroidobacterales ord. nov., and the species S. gossypii, S. soli, S. agariperforans, and S. agaridevorans are proposed to be moved to a novel genus Peristeroidobacter gen. nov. within the family Steroidobacteraceae.