Halomonas rituensis sp. nov. and Halomonas zhuhanensis sp. nov., isolated from natural salt marsh sediment on the Tibetan Plateau

A long-awaited taxogenomic investigation of the family Halomonadaceae

The family Halomonadaceae is 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 genus Halomonas is not phylogenetically consistent, likely because it has been used as a catch-all for newly described species within the family Halomonadaceae that could not be clearly accommodated in other Halomonadaceae genera. In the past decade, some taxonomic rearrangements have been conducted on the Halomonadaceae based 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 family Halomonadaceae; 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 of Halomonadaceae species 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 family Halomonadaceae, and the analysis of clade-specific signature genes. Based on our results, we conclude that the genus Halovibrio is obviously out of place within the family Halomonadaceae, and, on the other hand, we propose a division of the genus Halomonas into seven separate genera and the transfer of seven species from Halomonas to the genus Modicisalibacter, together with the emendation of the latter. Additionally, data from this study demonstrate the existence of various synonym species names in this family.

Halomonas flagellata sp. nov., a halophilic bacterium isolated from saline soil in Xinjiang

A Gram-stain-negative, strictly aerobic, motile, slightly curved rod-shaped bacterium with multiple flagella, designated strain EGI 63088T, was isolated from a bulk soil of Kalidium foliatum, collected from Wujiaqu in Xinjiang Uighur Autonomous Region, PR China. The optimal growth temperature, salinity, and pH for strain EGI 63088T growth were 30 °C, 3% (w/v) NaCl and 8, respectively. Phylogenetic analysis using 16S rRNA gene sequences indicated that strain EGI 63088T showed the highest sequence similarities to Halomonas heilongjiangensis 9-2T (97.94%), H. lysinitropha 3(2)T (97.51%), and H. daqiaonensis CGMCC 1.9150T (97.08%). The average nucleotide identity and digital DNA-DNA hybridization values between the strain EGI 63088T and H. heilongjiangensis 9-2T were 89.03 and 41.10%, respectively. The DNA G + C content of the genome for strain EGI 63088T was 66.3 mol%. The most prevalent antibiotic resistance and virulence-related genes in Halomonas genomes were Streptomyces cinnamoneu EF-Tu mutant, pilT, and cheY, respectively. The predominant fatty acids of strain EGI 63088T were summed feature 8 (C18: 1 ω6c and/or C18: 1 ω7c), summed feature 3 (C16: 1 ω6c and/or C16: 1 ω7c), and C16: 0; its major respiratory quinone was ubiquinone-9 (Q-9), and the major polar lipids were diphosphatidylglycerol, phosphatidylglycerol, and phosphatidylethanolamine. According to the above results, strain EGI 63088T is considered a novel species of the genus Halomonas, for which the name Halomonas flagellata sp. nov. is proposed. The type strain is EGI 63088T (= KCTC 92047T = CGMCC 1.19133T).

Larsenimonas rhizosphaerae sp. nov., isolated from a tidal mudflat

A Gram-reaction-negative, aerobic, motile, rod-shaped bacterium, designated GH3-8^T, was isolated from rhizosphere mudflats of halophytes on the seashore of Gangwha Island, Republic of Korea. Growth was observed at pH 4-10 (optimum, pH 7-8), at 4-40 °C (optimum, 37 °C) and in the presence of 0.5-20 % (w/v) NaCl (optimum, 4 %). The predominant respiratory quinone was Q-9. The major fatty acids were C_18 : 1 ω7c, C_16 : 0, summed feature 3 (C_16 : 1 ω7c and/or C_16 : 1 ω6c) and C_12 : 0 3OH. The polar lipids contained phosphatidylethanolamine, phosphatidylglycerol, an unidentified phosphoglycolipid, an unidentified phosphoglycoaminolipid, an unidentified glycoaminolipid, two unidentified phospholipids and two unidentified lipids. Phylogenetic analysis based on 16S rRNA gene sequences exhibited that the isolate belonged to the family Halomonadaceae, with the most closely related species, Larsenimonas suaedae (98.1 % sequence similarity) and Larsenimonas salina (97.9 %). Sequence similarity values between the isolate and other representatives of the family Halomonadaceae were all below 95.3 %. The values of average nucleotide identity between strain GH3-8^T and members of the genus Larsenimonas were 73.42 % with L. salina CCM 8464^T and 72.38 % with L. suaedae DSM 22428^T. Strain GH3-8^T showed digital DNA-DNA hybridization values of 18.5-18.6 % with members of the genus Larsenimonas. Based on phenotypic and chemotaxonomic distinctiveness together with low overall genomic relatedness indices and phylogenetic data, the isolate is considered to represent a new species of the genus Larsenimonas, for which the name Larsenimonas rhizosphaerae sp. nov. is proposed, with the type strain GH3-8^T (=KCTC 62127^T=NBRC 113214^T).