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Abstract Me.tha'no.thrix. N.L. neut. n. methanum methane; Gr. fem. n. thrix , hair; N.L. fem. n. Methanothrix , methane (‐producing) hair. Straight, rod‐shaped cells with flat ends, usually 0.8–1.3 μm wide by 2.0–6.0 μm long enclosed in a tubular sheath. Forms short (∼5–25 μm) to long (>150 μm) flexible chains of cells within the sheath. Nonmotile. Gram‐stain‐negative. Lipids contain myo ‐inositol, ethanolamine, and galactose as the polar head groups. Oxygen‐tolerant anaerobe. Organotrophic, splitting acetate into methane and CO 2 for energy generation. Some strains split formate into H 2 and CO 2 without producing methane. CO 2 can be reduced to methane in coculture with Geobacter spp. via direct interspecies electron transfer (DIET). Growth factors such as vitamins are stimulatory. Yeast extract is required, stimulatory or inhibitory, depending on the strain. NaCl is not required for growth. Optimal temperatures range from 34 to 37°C for mesophilic strains and 55 to 60°C for thermophilic strains; optimal pH range is 7.0–7.8. Gas vacuoles are generally found in thermophilic strains. Occur in both mesophilic and thermophilic anaerobic sludge digesters as well as anaerobic sediments. Synonymous with the genus Methanosaeta . DNA G + C content (mol%) : 51–61 (genome). Type species : Methanothrix soehngenii Huser et al. 1982, VL10. Taxonomic and Nomenclature Notes According to the List of Prokaryotic names with Standing in Nomenclature (LPSN), the taxonomic status of the genus Methanothrix is: correct name (last update, February 2025) * . LPSN classification: Archaea / Methanobacteriati / Methanobacteriota / Methanosarcinia / Methanosarcinales / Methanotrichaceae / Methanothrix The genus Methanothrix can also be recovered in the Genome Taxonomy Database (GTDB) as g__Methanothrix (version v220) ** . GTDB classification: d__Archaea / p__Halobacteriota / c__Methanosarcinia / o__Methanotrichales / f__Methanotrichaceae / g__Methanothrix * Meier‐Kolthoff et al. ( 2022 ). Nucleic Acids Res , 50 , D801 – D807 ; DOI: 10.1093/nar/gkab902 ** Parks et al. ( 2022 ). Nucleic Acids Res , 50 , D785 – D794 ; DOI: 10.1093/nar/gkab776

Abstract Ha.lo.re.di.vi'vus. Gr. masc. n. hals , halos salt; L. masc. adj. redivivus renewed, renovated; N.L. masc. n. Haloredivivus a reconstructed salty organism. The genus Candidatus Haloredivivus was established based on a sequence of a single‐cell‐amplified genome recovered from a solar saltern pond of intermediate salinity in Spain. Ca . Haloredivivus is phylogenetically affiliated with members of the Candidatus Nanohaloarchaeota branch ( Candidatus Nanosalina, Candidatus Nanosalinicola corrig., and Candidatus Nanopetraeus corrig.) of the archaeal DPANN superphylum. Analysis of the ∼1.2 Mb genome of Ca . Haloredivivus with 42.1% G + C suggests that the organism leads a photoheterotrophic lifestyle and may be able to degrade different polysaccharides. The low median isoelectric point for the predicted proteins suggests a “salt‐in” strategy for osmotic balance. Gene sequences affiliated with Ca . Haloredivivus were recovered from a salt lake in Romania and from soda lakes in Siberia. DNA G + C content (mol%) : 42.0–42.1 (genome sequence). Type species : Not assigned. Taxonomic and Nomenclature Notes According to the List of Prokaryotic names with Standing in Nomenclature (LPSN), the taxonomic status of the genus Candidatus Haloredivivus is: preferred name (not correct name) (last update, February 2025) * . LPSN classification: Archaea / Nanobdellati / Candidatus Nanohalarchaeota / Candidatus Nanohaloarchaea / incertae sedis / incertae sedis / Candidatus Haloredivivus Candidatus Haloredivivus could not be recovered in GTDB ** . * Meier‐Kolthoff et al. ( 2022 ). Nucleic Acids Res , 50 , D801 – D807 ; DOI: 10.1093/nar/gkab902 ** Parks et al. ( 2022 ). Nucleic Acids Res , 50 , D785 – D794 ; DOI: 10.1093/nar/gkab776

Abstract Na.no.sa.li'na. Gr. masc. n. nanos dwarf; N.L. masc. adj. salinus salty; N.L. fem. n. Nanosalina a dwarf‐size salty organism. The genus Candidatus Nanosalina was established based on a sequence from a metagenome recovered from the hypersaline Lake Tyrrell in Victoria, Australia. Ca . Nanosalina is phylogenetically affiliated with members of the Candidatus Nanohaloarchaeota branch ( Candidatus Nanosalinicola corrig., Candidatus Nanopetraeus corrig., and Candidatus Haloredivivus) of the archaeal DPANN superphylum. On the basis of fluorescence in situ hybridization, cells are ∼0.6 μm in diameter. Analysis of the ∼1.23 Mb genome of Ca . Nanosalina J07AB43 with 43.5% DNA G + C suggests that the organism possesses an aerobic, photoheterotrophic lifestyle. The high proportion of acidic amino acids of the predicted proteins suggests a “salt‐in” strategy for osmotic balance. Gene sequences affiliated with Ca . Nanosalina have been recovered from coastal salterns in Australia and California, USA. DNA G + C content (mol%) : 43.5 (genome sequence). Type species : Not assigned. Taxonomic and Nomenclature Notes According to the List of Prokaryotic names with Standing in Nomenclature (LPSN), the taxonomic status of the genus Candidatus Nanosalina is: preferred name (not correct name) (last update, February 2025) * . LPSN classification: Archaea / Nanobdellati / Candidatus Nanohalarchaeota / Candidatus Nanohaloarchaea / Candidatus Nanosalinales / Candidatus Nanosalinaceae / Candidatus Nanosalina Candidatus Nanosalina could not be recovered in GTDB ** . * Meier‐Kolthoff et al. ( 2022 ). Nucleic Acids Res , 50 , D801 – D807 ; DOI: 10.1093/nar/gkab902 ** Parks et al. ( 2022 ). Nucleic Acids Res , 50 , D785 – D794 ; DOI: 10.1093/nar/gkab776

Abstract Na.no.sa.li.ni'co.la. Gr. masc. n. nanos dwarf; L. fem. pl. n. salinae saltworks; L. suff. –cola (from L. masc. or fem. n. incola ) inhabitant, dweller; N.L. masc. n. Nanosalinicola a dwarf‐sized organism inhabiting saltworks. The genus Candidatus Nanosalinicola corrig., originally described as Candidatus Nanosalinarum, was established based on a sequence from a metagenome recovered from the hypersaline Lake Tyrrell in Victoria, Australia. Ca . Nanosalinicola is phylogenetically affiliated with members of the Candidatus Nanohaloarchaeota branch ( Candidatus Nanosalina, Candidatus Nanopetraeus corrig., and Candidatus Haloredivivus) of the archaeal DPANN superphylum. On the basis of fluorescence in situ hybridization, cells are ∼0.6 μm in diameter. Analysis of the ∼1.22 Mb genome of Ca . Nanosalinicola J07AB56 with 56% DNA G + C suggests that the organism is an aerobic photoheterotroph. The high proportion of acidic amino acids of the predicted proteins suggests a “salt‐in” strategy for osmotic balance. Gene sequences affiliated with Ca . Nanosalinicola have also been recovered from coastal salterns in Australia and California, USA. DNA G + C content (mol%) : 56 (genome sequence). Type species : Not assigned. Genome sequence : AEIX00000000. Taxonomic and Nomenclature Notes According to the List of Prokaryotic names with Standing in Nomenclature (LPSN), the taxonomic status of the genus Candidatus Nanosalinicola is: preferred name (not correct name) (last update, February 2025) * . LPSN classification: Archaea / Nanobdellati / Candidatus Nanohalarchaeota / Candidatus Nanohaloarchaea / incertae sedis / incertae sedis / Candidatus Nanosalinicola Candidatus Nanosalinicola could not be recovered in GTDB ** . * Meier‐Kolthoff et al. ( 2022 ). Nucleic Acids Res , 50 , D801 – D807 ; DOI: 10.1093/nar/gkab902 ** Parks et al. ( 2022 ). Nucleic Acids Res , 50 , D785 – D794 ; DOI: 10.1093/nar/gkab776

A novel obligately anaerobic, thermophilic and formate-utilizing bacterium K32T was isolated from Shengli oilfield of China. Cells were straight rods (0.4–0.8 µm × 2.5–8.0 µm), Gram-stain-positive, non-spore-forming and slightly motile. Optimum growth occurred with pH of 7 and 0.5 g l–1 NaCl under temperature of 55–60 °C. Nitrate could be reduced into nitrite, syntrophic formate oxidation to methane and carbon dioxide occurred when co-culturing strain K32T and Methanothermobacter thermautotrophicus ΔH. The main cellular fatty acids were iso-C15 : 0 (24.0 %), anteiso-C15 : 0 (21.7 %), C16 : 0 (12.7 %) and C14 : 0 (10.8 %), and the main polar lipid was phosphatidylglycerol. The G+C content of the genomic DNA was 46.3 mol%. The 16S rRNA gene sequence of K32T shared ≤90.4 % of sequence similarity to closest type strains of Desulfitibacter alkalitolerans , Calderihabitans maritimus and members of the genus Moorella . Based on the phenotypic, biochemical and genotypic characterization, Zhaonella formicivorans gen. nov., sp. nov. is proposed with K32T (=CCAM 584T =DSM 107278T=CGMCC1.5297T) as the type strain, which is the first representative of Zhaonellaceae fam. nov. In addition, the order Thermoanaerobacterales and family Peptococcaceae were reclassified, and three novel families in the novel order of Moorellales ord. nov. were also proposed.

A novel endophytic actinobacterium, designated strain EGI 650086T, was isolated from the roots of Anabasis elatior (C.A.Mey.) Schischk. collected in Xinjiang, north-west China. The taxonomic position of the strain was investigated using a polyphasic taxonomic approach. Growth occurred at 15–40 °C, pH 6.0–8.0 and in the presence of 0–6 % NaCl (w/v). Phylogenetic analysis based on 16S rRNA gene sequence and concatenation of 22 protein marker genes revealed that strain EGI 650086T formed a monophyletic clade within the genus Amycolatopsis and shared the highest sequence similarities with Amycolatopsis nigrescens JCM 14717T (97.1 %) and Amycolatopsis sacchari DSM 44468T (97.0 %). Sequence similarities with type strains of other species of the genus Amycolatopsis were less than 97.0 %. The average nucleotide identity and DNA–DNA hybridization values between strain EGI 650086T and the reference strains were 78.1–79.8 % and 22.1–23.0 %, respectively. The genome of strain EGI 650086T was 10.9 Mb, with a DNA G+C content of 70.1 mol%. The diagnostic diamino acid in the peptidoglycan was meso-diaminopimelic acid. The major whole-cell sugars contained arabinose, galactose, glucose and ribose. The predominant menaquinones were MK-9 (H4) and MK-9 (H2). Major fatty acids were iso-C16 : 0 and summed feature 4 (iso-C17 : 1 I and/or anteiso-C17 : 1 B). The polar lipid profile of strain EGI 650086T included diphosphatidylglycerol, phosphatidylglycerol, phosphatidylethanolamine, hydroxy-phosphatidylethanolamine, phosphatidylinositol, phosphatidylinositol mannosides, two unknown phospholipids, an unknown glycolipid and an unknown lipid. Polyphasic taxonomic characteristics indicated that strain EGI 650086T represents a novel species of the genus Amycolatopsis , for which the name Amycolatopsis anabasis sp. nov. is proposed. The type strain is EGI 650086T (=KCTC 49044T=CGMCC 4.7188T).

A novel actinobacterial strain, designated K81G1T, was isolated from a soil sample collected in Kantulee peat swamp forest, Surat Thani Province, Thailand, and its taxonomic position was determined using a polyphasic approach. Optimal growth of strain K81G1Toccurred at 28–30 °C, at pH 5.0–6.0 and without NaCl. Strain K81G1Thad cell-wall chemotype IV (meso-diaminopimelic acid as the diagnostic diamino acid, and arabinose and galactose as diagnostic sugars) and phospholipid pattern type II, characteristic of the genusAmycolatopsis. It contained MK-9(H4) as the predominant menaquinone, iso-C16 : 0, C17 : 0cyclo and C16 : 0as the major cellular fatty acids, and phospholipids consisting of phosphatidylglycerol, diphosphatidylglycerol, phosphatidylethanolamine, hydroxyphosphatidylethanolamine, phosphatidylinositol and two unidentified phospholipids. Based on 16S rRNA gene sequence similarity and phylogenetic analyses, strain K81G1Twas most closely related toAmycolatopsis rhizosphaeraeTBRC 6029T(97.8 % similarity),Amycolatopsis acidiphilaJCM 30562T(97.8 %) andAmycolatopsis bartoniaeDSM 45807T(97.6 %). Strain K81G1Texhibited low average nucleotide identity and digital DNA–DNA hybridization values withA. rhizosphaeraeTBRC 6029T(76.4 %, 23.0 %),A. acidiphilaJCM 30562T(77.9 %, 24.6 %) andA. bartoniaeDSM 45807T(77.8 %, 24.3 %). The DNA G+C content of strain K81G1Twas 69.7 mol%. Based on data from this polyphasic study, strain K81G1Trepresents a novel species of the genusAmycolatopsis, for which the nameAmycolatopsis acidicolasp. nov. is proposed. The type strain is K81G1T(=TBRC 10047T=NBRC 113896T).

A novel Gram-stain-positive, rod shaped and anaerobic bacterium, designated as W6T, was isolated from Shengli oilfield in China. Strain W6T was observed to grow from 20 to 45 °C with pH 6.5–9.0 (optimally at 40 °C and pH of 7.5) and without addition of NaCl. The major cellular fatty acids were iso-C15 : 0 (29.1%), C14 : 0 (27.0%) and C16 : 0 (12.2%), and the main polar lipids were lipids (L) and aminolipids (AL). The DNA G+C content is 42.9 mol%. Based on 16S rRNA gene sequence analysis, strain W6T showed highest similarities to Tissierella creatinini DSM 9508T (94.9%) and Soehngenia saccharolytica DSM 12858T (94.1%). The morphological, physiological, biochemical, phylogenetic and phylogenomic analyses demonstrated strain W6T (CCAM 534T=DSM 28124T=CGMCC 1.5291T) represents a novel species in a new genus, for which the name Gudongella oleilytica gen. nov. sp. nov. is proposed. The family Tissierellaceae is proposed as a new family containing the genera Anaerosalibacter , Gudongella, Keratinibaculum , Soehngenia , Sporanaerobacter , Tepidimicrobium , Tissierella , Urmitella and species Clostridium ultunense based on the phylogenetic and phylogenomic analyses.

Genome analysis is one of the main criteria for description of new taxa. Availability of genome sequences for all the actinobacteria with a valid nomenclature will, however, require another decade’s works of sequencing. This paper describes the rearrangement of the higher taxonomic ranks of the members of the phylum ‘ Actinobacteria ’, using the phylogeny of 16S rRNA gene sequences and supported by the phylogeny of the available genome sequences. Based on the refined phylogeny of the 16S rRNA gene sequences, we could arrange all the members of the 425 genera of the phylum ‘ Actinobacteria ’ with validly published names currently in use into six classes, 46 orders and 79 families, including 16 new orders and 10 new families. The order Micrococcales Prévot 1940 (Approved Lists 1980) emend. Nouioui et al. 2018 is now split into 11 monophyletic orders: the emended order Micrococcales and ten proposed new orders Aquipuribacterales , Beutenbergiales , Bogoriellales , Brevibacteriales , Cellulomonadales , Demequinales , Dermabacterales , Dermatophilales , Microbacteriales and Ruaniales . Further, the class ‘ Actinobacteria ’ Stackebrandt et al. 1997 emend. Nouioui et al. 2018 was described without any nomenclature type, and therefore the name ‘ Actinobacteria ’ is deemed illegitimate. In accordance to Rule 8 of the International Code of Nomenclature of Prokaryotes, Parker et al. 2019, we proposed the name Actinomycetia which is formed by using the stem of the name Actinomycetales Buchanan 1917 (Approved Lists 1980) emend. Zhi et al. 2009, to replace the name ‘ Actinobacteria ’. The nomenclature type of the proposed new class Actinomycetia is the order Actinomycetales Buchanan 1917 (Approved Lists 1980) emend. Zhi et al. 2009.