A taxonomic study was carried out on strains 22II1-22F38(T) and 22II-S13e, which were isolated from sea water and sediment from the Atlantic Ocean, respectively. The two strains were Gram-negative, oxidase and catalase positive, oval to pear shaped, and motile by a single polar flagellum. Phylogenetic analysis based on 16S rRNA gene sequences indicated that both strains belonged to the genus Hyphomonas, with highest sequence similarity (98.2%) to the type strains H. jannaschiana DSM 5153(T) and H. johnsonii ATCC 43964(T). The genomic ANIm values and DNA-DNA hybridization estimate values between strain 22II1-22F38(T) and seven type strains ranged from 82.84% to 84.10% and from 18.0% to 19.1%, respectively. Isolate 22II1-22F38(T) had a G+C content of 58.3% and used Q-11 as the predominant respiratory quinone. The combined phenotypic and genotypic data showed that both strains represented a novel species of the genus Hyphomonas, for which the name Hyphomonas atlanticus sp. nov. is proposed, with the type strain being 22II1-22F38(T) (=LMG 27916(T)=MCCC 1A09418(T)). In addition, we conclude that Hyphomonas hirschiana is a later synonym of Hyphomonas neptunium.

Hyphomonas neptunium is a marine prosthecate alpha-proteobacterium currently classified as a member of the order 'Rhodobacterales'. Although this classification is supported by 16S rRNA gene sequence phylogeny, 23S rRNA gene sequence analysis, concatenated ribosomal proteins, HSP70 and EF-Tu phylogenies all support classifying Hyphomonas neptunium as a member of the Caulobacterales instead. The possible reasons why the 16S rRNA gene sequence gives conflicting results in this case are also discussed.