Stephane L'Haridon

A novel hyperthermophilic, heterotrophic and anaerobic archaeon, designated strain SLHALR2 ^T , was isolated from the ‘Cooking Pots’ hot springs at Hell’s Gate, Tikitere, New Zealand. During the exponential growth phase, cells of strain SLHALR2 ^T appear as long, thin rods to filaments, measuring 0.25–0.4 µm in width and 3.0–7.0 µm in length. Strain SLHALR2 ^T grows at temperatures ranging from 60 to 97 °C (optimum 80–85 °C), at pH values between 5.0 and 7.3 (optimum pH 6.0) and with 0–8 g l ⁻¹ sodium chloride concentration (optimum 0–1.0 g l ⁻¹ ). Elemental sulphur is reduced to hydrogen sulphide. Strain SLHALR2 ^T is an obligate organotroph capable of utilizing a variety of proteinaceous substrates and a limited range of sugars. Archaeal cell extracts improve growth but are not mandatory for growth. Phylogenetic analysis based on 16S rRNA gene sequence comparisons indicates that strain SLHALR2 ^T belongs to the genus Infirmifilum within the family Thermofilaceae , order Thermofilales (phylum Thermoproteota ). The strain shares 98.4–99% 16S rRNA gene sequence similarity with described Infirmifilum species. Despite this high level of 16S rRNA gene similarity with recently described Infirmifilum species, both average nucleotide identity (71–74%), average amino acid identity (67–75.5%) values and digital DNA–DNA hybridization values support the designation of strain SLHALR2 ^T as a distinct species within Infirmifilum . Based on phylogenomic analyses and the physiological characteristics of the novel isolate, strain SLHALR2 ^T is proposed as the type strain of a new species, Infirmifilum tikiterense sp. nov. (=OCM2026=UBOCC M-3571), and an emended description of the genus Infirmifilum is proposed.

Active hydrothermal chimneys were sampled from the newly discovered La Scala Vent Field, located on the Woodlark Ridge in the Pacific Ocean. Enrichment cultures yielded hyperthermophilic methanogens, from which several strains were isolated. Phylogenetic analysis based on the 16S rRNA gene and genome sequences revealed that strain 28A ^T belongs to the genus Methanocaldococcus , within the family Methanocaldococcaceae and the order Methanococcales . Strain 28A ^T shared 95.4–99.1% 16S rRNA gene sequence similarity with other species of Methanocaldococcus and was most closely related to Methanocaldococcus bathoardescens . Cells of strain 28A ^T were coccoid in shape, with diameters ranging from 1.0 to 3.0 µm. The strain grew at temperatures between 50 and 85 °C (optimum: 75–80 °C), at pH values between 5.3 and 7.0 (optimum: pH 6.5) and in sodium chloride (NaCl) concentrations ranging from 5 to 50 g l⁻¹ (optimum: 20 g l⁻¹). Genome analysis revealed the presence of a 3.9 kbp plasmid. Based on 16S rRNA and genome phylogenetic analysis, average nucleotide identity and digital DNA–DNA hybridization values, we propose that strain 28A ^T represents a novel species, for which the name Methanocaldococcus abyssi sp. nov. is proposed. The type strain is 28Aᵀ (=DSM 120179ᵀ=JCM 39640ᵀ).

Methanogenic archaea are a diverse, polyphyletic group of strictly anaerobic prokaryotes capable of producing methane as their primary metabolic product. It has been over three decades since minimal standards for their taxonomic description have been proposed. In light of advancements in technology and amendments in systematic microbiology, revision of the older criteria for taxonomic description is essential. Most of the previously recommended minimum standards regarding phenotypic characterization of pure cultures are maintained. Electron microscopy and chemotaxonomic methods like whole-cell protein and lipid analysis are desirable but not required. Because of advancements in DNA sequencing technologies, obtaining a complete or draft whole genome sequence for type strains and its deposition in a public database are now mandatory. Genomic data should be used for rigorous comparison to close relatives using overall genome related indices such as average nucleotide identity and digital DNA–DNA hybridization. Phylogenetic analysis of the 16S rRNA gene is also required and can be supplemented by phylogenies of the mcrA gene and phylogenomic analysis using multiple conserved, single-copy marker genes. Additionally, it is now established that culture purity is not essential for studying prokaryotes, and description of Candidatus methanogenic taxa using single-cell or metagenomics along with other appropriate criteria is a viable alternative. The revisions to the minimal criteria proposed here by the members of the Subcommittee on the Taxonomy of Methanogenic Archaea of the International Committee on Systematics of Prokaryotes should allow for rigorous yet practical taxonomic description of these important and diverse microbes.

Three hyperthermophilic methanogens, designated strain SG7^T, strain SG1 and strain SLH, were isolated from the ABE and Tu’i Malila deep-sea hydrothermal vent fields along the Eastern Lau Spreading Center. Phylogenetic analysis based on 16S rRNA gene sequence indicated that strains SG7^T, SG1 and SLH were affiliated with the genusMethanocaldococcuswithin the familyMethanocaldococcaceae, orderMethanococcales. They shared 95.5–99.48 % 16S rRNA gene sequence similarity to otherMethanocaldococcusspecies and were most closely related toMethanocaldococcus bathoardescens. Cells of strains SG7^T, SG1 and SLH were cocci, with a diameter of 1.0–2.2 µm. The three strains grew between 45 and 93 °C (optimum, 80–85 °C), at pH 5.0–7.1 (optimum pH 6.2) and with 10–50 g l⁻¹NaCl (optimum 20–25 g l⁻¹). Genome analysis revealed the presence of a 5.1 kbp plasmid in strain SG7^T. Based on the results of average nucleotide identity and digital DNA–DNA hybridization analyses, we propose that strains SG1 and SG7^Tare representatives of a novel species, for which the nameMethanocaldococcus lauensissp. nov. is proposed; the type strain is SG7^T(=DSM 109608^T=JCM 39049^T).

Fer.vi.do.bac.te'ri.um. L. masc. adj. fervidus hot: Gr. neut. n. bakterion a small rod: N.L. neut. n. Fervidobacterium a rod that grow at higher temperatures. The genus Fervidobacterium is currently composed of seven described species ( Fervidobacterium changbaicum , Fervidobacterium gondwanense , Fervidobacterium islandicum , Fervidobacterium nodosum , Fervidobacterium pennivorans , Fervidobacterium riparium , and Fervidobacterium thailandense ). They are members of the phylum Thermotogota , class Thermotogae , order Thermotogales , and family Fervidobacteriaceae . Similar to other Thermotogota , Fervidobacterium spp. are obligately anaerobic heterotrophs . They are thermophilic or hyperthermophilic , with optimum growth temperature of 60–80°C and optimal pH of 6.5–7.8. However, unlike other described Thermotogota , Fervidobacterium spp. grow only at low salinity (0–0.5% NaCl). Sulfur is reduced to H ₂ S and stimulates growth, while molecular hydrogen inhibits growth. The cells are Gram‐negative rods and occur individually, in pairs, in filaments, in chains of up to 17 cells in one chain, or as spherical rotund bodies where 10 or more cells share an outer membrane. The majority of the individual cells produce terminal protuberances on one end of the cell (spheroids). Cells size about 0.4–0.6 × 1–40 μm. The cells are sensitive to inhibitors of cell‐wall, protein, and nucleic acid biosynthesis. Known habitats are terrestrial geothermal systems. The genus has also been detected in several types of anthropogenic environments: oil sands, tailing ponds, effluent of pulp and paper industry, and thermophilic microbial fuel cells. Several isolates have important uses in biotechnology; for instance, keratinophilic isolates are known to degrade feathers. DNA G + C content (mol%) : 35–45.8. Type species : Fervidobacterium nodosum Patel et al. 1985, VL19. Taxonomic and Nomenclature Notes: According to the List of Prokaryotic names with Standing in Nomenclature (LPSN), the taxonomic status of the genus Fervidobacterium is: correct name (last update, February 2025) ^* . LPSN classification: Bacteria / Thermotogati / Thermotogota / Thermotogae / Thermotogales / Fervidobacteriaceae / Fervidobacterium The genus Fervidobacterium can also be recovered in the Genome Taxonomy Database (GTDB) as g__Fervidobacterium (version v220) ^** . GTDB classification: d__Bacteria / p__Thermotogota / c__Thermotogae / o__Thermotogales / f__Fervidobacteriaceae / g__Fervidobacterium ^* 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 bacterial strain, designated BAR1^T, was isolated from a microbial mat growing on the surface of a barite chimney at the Loki’s Castle Vent Field, at a depth of 2216 m. Cells of strain BAR1^T were rod-shaped, Gram-reaction-negative and grew on marine broth 2216 at 10–37 °C (optimum 27–35 °C), pH 5.5–8.0 (optimum pH 6.5–7.5) and 0.5–5.0 % NaCl (optimum 2 %). The DNA G+C content was 57.38 mol%. The membrane-associated major ubiquinone was Q-10, the fatty acid profile was dominated by C_18 : 1ω7c (91 %), and the polar lipids detected were phosphatidylcholine, phosphatidylglycerol, phosphatidylethanolamine, one unidentified aminolipid, one unidentified lipid and one unidentified phospholipid. Phylogenetic analyses based on 16S rRNA gene sequences showed that strain BAR1^T clustered together with Rhodobacterales bacterium PRT1, as well as the genera Halocynthiibacter and Pseudohalocynthiibacter in a polyphyletic clade within the Roseobacter clade. Several characteristics differentiate strain BAR1^T from the aforementioned genera, including its motility, its piezophilic behaviour and its ability to grow at 35 °C and under anaerobic conditions. Accordingly, strain BAR1^T is considered to represent a novel genus and species within the Roseobacter clade, for which the name Profundibacter amoris gen. nov., sp. nov. is proposed. The type strain is Profundibacter amoris BAR1^T (=JCM 31874^T=DSM 104147^T).