Halophilic archaea of the class Halobacteria are the most salt-requiring prokaryotes within the domain Archaea. In 1997, minimal standards for the description of new taxa in the order Halobacteriales were proposed. From then on, the taxonomy of the class Halobacteria provides an excellent example of how changing concepts on prokaryote taxonomy and the development of new methods were implemented. The last decades have witnessed a rapid expansion of the number of described taxa within the class Halobacteria coinciding with the era of genome sequencing development. The current members of the International Committee on Systematics of Prokaryotes Subcommittee on the Taxonomy of Halobacteria propose these revisions to the recommended minimal standards and encourage the use of advanced technologies in the taxonomic description of members of the Halobacteria. Most previously required and some recommended minimal standards for the description of new taxa in the class Halobacteria were retained in the present revision, but changes have been proposed in line with the new methodologies. In addition to the 16S rRNA gene, the rpoB′ gene is an important molecular marker for the identification of members of the Halobacteria. Phylogenomic analysis based on concatenated conserved, single-copy marker genes is required to infer the taxonomic status of new taxa. The overall genome relatedness indexes have proven to be determinative in the classification of the taxa within the class Halobacteria. Average nucleotide identity, digital DNA–DNA hybridization, and average amino acid identity values should be calculated for rigorous comparison among close relatives.

Four halophilic archaeal strains, YPL8^T, SLN56^T, LT61^T and KZCA68^T, were isolated from a salt mine, saline soil and a salt lake located in different regions of China. Sequence similarities of 16S rRNA and rpoB′ genes among strains YPL8^T, SLN56^T, LT61^T and the current members of Natrinema were 94.1–98.2 % and 89.3–95.1 %, respectively, while these values among strain KZCA68^T and the current members of Haloterrigena were 97.2–97.4 % and 91.7–91.9 %, respectively. The average nucleotide identity, in silico DNA–DNA hybridization and average amino acid identity values among these four strains and their closely related species were all lower than the threshold values for species boundary. All four strains were unable to hydrolyse casein, gelatin, or Tween 80. Strain YPL8^T contained phosphatidic acid (PA), phosphatidylglycerol (PG), phosphatidylglycerol phosphate methyl ester (PGP-Me), sulfated mannosyl glucosyl diether (S-DGD-1), disulfated mannosyl glucosyl diether (S₂-DGD) and sulfated mannosyl glucosyl diether-phosphatidic acid (S-DGD-PA). Strain SLN56^T contained PA, PG, phosphatidylglycerol sulphate (PGS), PGP-Me, S-DGD-1, S₂-DGD and S-DGD-PA. Strain LT61^T contained PA, PG, PGS, PGP-Me, S-DGD-1 and S₂-DGD. The phospholipids of strain KZCA68^T were PA, PG and PGP-Me. These results showed that strains YPL8^T (=CGMCC 1.13883^T=JCM 31181^T), SLN56^T (=CGMCC 1.14945^T=JCM 30832^T) and LT61^T (=CGMCC 1.14942^T=JCM 30668^T) represent novel species of the genus Natrinema , for which the names, Natrinema halophilum sp. nov., Natrinema salinisoli sp. nov. and Natrinema amylolyticum sp. nov. are proposed. Strain KZCA68^T (=CGMCC 1.17211^T=JCM 34158^T) represents a novel species of Haloterrigena , for which the name Haloterrigena alkaliphila sp. nov. is proposed.

Two novel halophilic archaeal strains, Gai3-17^T and XZYJT26^T, were isolated from the sediment of Gaize salt lake and the saline soil of Mangkang ancient solar saltern in Tibet, PR China, respectively. Strains Gai3-17^T and XZYJT26^T were related to each other (96.5 and 89.7% similarity, respectively) and showed 97.5–95.4 and 91.5–87.7% similarities to the current members of Halobacterium based on 16S rRNA and rpoB′ genes. The phylogenomic analysis indicated that strains Gai3-17^T and XZYJT26^T formed two distinct clades and clustered with the Halobacterium species. The two strains can be differentiated from the type strains of the six species with validly published names based on several phenotypic characteristics. The phospholipids of the two strains were phosphatidic acid, phosphatidylglycerol and phosphatidylglycerol phosphate methyl ester. One major glycolipid, sulphated galactosyl mannosyl glucosyl diether, was detected in strain Gai3-17^T, while four glycolipids, mannosyl glucosyl diether, sulphated mannosyl glucosyl diether, disulphated mannosyl glucosyl diether and sulphated galactosyl mannosyl glucosyl diether were observed in strain XZYJT26^T. The average nucleotide identity, digital DNA–DNA hybridization and amino acid identity values among the two strains and the members of Halobacterium were no more than 81, 25 and 77 %, respectively. These overall genome-related indices were below the threshold values for species boundary, indicating that strains Gai3-17^T and XZYJT26^T represent two novel species of Halobacterium . Thus, two novel species, Halobacterium wangiae sp. nov. and Halobacterium zhouii sp. nov., are proposed to accommodate strains Gai3-17^T (=CGMCC 1.16101^T=JCM 33551^T) and XZYJT26^T (=CGMCC 1.16682^T=JCM 33556^T), respectively.

Two halophilic archaeal strains, C90^T and YPL13^T, were isolated from a salt lake and a salt mine in PR China. The two strains were found to form two clusters (97.5 and 89.5 % similarity between them, respectively) separating them from the three current members of the genus Natronomonas (95.4–97.0 % and 86.6–89.3 % similarity, respectively) on the basis of the 16S rRNA and rpoB′ gene sequence similarities and phylogenetic analysis. Diverse phenotypic characteristics differentiate strains C90^T and YPL13^T from current Natronomonas members. The polar lipids of strain C90^T were phosphatidic acid, phosphatidylglycerol (PG), phosphatidylglycerol phosphate methyl ester (PGP-Me), phosphatidylglycerol sulphate, two unidentified glycolipids, a major glycolipid and a minor glycolipid, while those of strain YPL13^T were PG, PGP-Me, two unidentified phospholipids and a glycolipid. The average nucleotide identity (ANI) and in silico DNA–DNA hybridization (isDDH) values between the two strains were 79.8 and 27.1 %, respectively, which were much lower than the threshold values proposed as a species boundaries (ANI 95–96 % and isDDH 70 %), which revealed that the two strains represent two novel species; these values (ANI 76.6–80.0 % and isDDH 21.6–27.0 %) of the strains examined in this study and the current members of Natronomonas are much lower than the recommended threshold values, suggesting that strains C90^T and YPL13^T represent two genomically different species of Natronomonas . These results showed that strains C90^T (=CGMCC 1.13738^T=JCM 32961^T) and YPL13^T (=CGMCC 1.13884^T=JCM 31111^T) represent two novel species of Natronomonas , for which the names Natronomonas halophila sp. nov. and Natronomonas salina sp. nov. are proposed.