Sánchez-Porro, Cristina

The familyHalomonadaceaeis the largest family composed of halophilic bacteria, with more than 160 species with validly published names as of July 2023. Several classifications to circumscribe this family are available in major resources, such as those provided by the List of Prokaryotic names with Standing in Nomenclature (LPSN), NCBI Taxonomy, Genome Taxonomy Database (GTDB), and Bergey’s Manual of Systematics of Archaea and Bacteria (BMSAB), with some degree of disagreement between them. Moreover, regardless of the classification adopted, the genusHalomonasis not phylogenetically consistent, likely because it has been used as a catch-all for newly described species within the familyHalomonadaceaethat could not be clearly accommodated in otherHalomonadaceaegenera. In the past decade, some taxonomic rearrangements have been conducted on theHalomonadaceaebased on ribosomal and alternative single-copy housekeeping gene sequence analysis. High-throughput technologies have enabled access to the genome sequences of many type strains belonging to the familyHalomonadaceae; however, genome-based studies specifically addressing its taxonomic status have not been performed to date. In this study, we accomplished the genome sequencing of 17 missing type strains ofHalomonadaceaespecies that, together with other publicly available genome sequences, allowed us to re-evaluate the genetic relationship, phylogeny, and taxonomy of the species and genera within this family. The approach followed included the estimate of the Overall Genome Relatedness Indexes (OGRIs) such as the average amino acid identity (AAI), phylogenomic reconstructions using amino acid substitution matrices customized for the familyHalomonadaceae, and the analysis of clade-specific signature genes. Based on our results, we conclude that the genusHalovibriois obviously out of place within the familyHalomonadaceae,and, on the other hand, we propose a division of the genusHalomonasinto seven separate genera and the transfer of seven species fromHalomonasto the genusModicisalibacter, together with the emendation of the latter. Additionally, data from this study demonstrate the existence of various synonym species names in this family.

The discovery of the new halophilic archaeon, Halorutilus salinus , representing a novel order, family, genus, and species within the class Halobacteria and phylum Euryarchaeota clearly enables insights into the microbial dark matter, expanding the current taxonomical knowledge of this group of archaea. The in-depth comparative genomic analysis performed on this new taxon revealed one of the first known examples of an Halobacteria representative coding the archaeal RuBisCO gene and with a streamlined genome, being ecologically successful in nature and explaining its previous non-isolation.