A novel strictly anaerobic, halotolerant, organotrophic bacterium, strain P3M-3^T, was isolated from a microbial mat formed under the flow of hot water emerging from a 2775 m-deep well in Tomsk region (western Siberia, Russia). Cells of strain P3M-3^T were straight and curved rods, 0.2–0.4 µm in width and 1.5–20 µm in length. Strain P3M-3^T grew optimally at 37 °C, pH 7.0–7.5 and in a NaCl concentration of 15 g l⁻¹. Under optimum growth conditions, the doubling time was 1 h. The isolate was able to ferment a variety of mono-, di- and polysaccharides, including microcrystalline cellulose. Acetate, ethanol, H₂ and CO₂ were the main products of glucose fermentation. The DNA G+C content was 33.4 mol%. 16S rRNA gene-based phylogenetic analysis showed that strain P3M-3^T was a member of family Lachnospiraceae , whose representatives are also found in Clostridium cluster XIVa. 16S rRNA gene sequence similarity with Clostridium jejuense HY-35-12^T, the closest relative, was 93.9 %. A novel genus and species, Mobilitalea sibirica gen. nov., sp. nov., are proposed based on phylogenetic analysis and physiological properties of the novel isolate. The type strain of the type species is P3M-3^T ( = DSM 26468^T = VKM B-2804^T).

A novel obligately anaerobic, extremely thermophilic, organotrophic bacterium, strain ik275mar^T, was isolated from a Mid-Atlantic Ridge deep-sea hydrothermal vent. Cells were rods surrounded by a sheath-like structure (toga), 0.4–0.9 µm in width and 1.2–6.0 µm in length. Strain ik275mar^Tgrew at 37–75 °C, pH 5.6–8.2 and at NaCl concentrations of 10–55 g l⁻¹. Under optimum conditions (70 °C, pH 6.6, NaCl 20 g l⁻¹), doubling time was 32 min. The isolate was able to ferment carbohydrates including starch, cellulose and cellulose derivatives. Acetate, H₂and CO₂were the main products of glucose fermentation. G+C content of DNA was 27 mol%. Phylogenetic analysis of 16S rRNA gene sequences showed that strain ik275mar^Tis a member of the genusThermosipho. 16S rRNA gene sequence identity with the other species of the genusThermosiphoranged from 93.7 to 94.5 %. Based on the phylogenetic analysis and physiological properties of the novel isolate, we propose a novel species,Thermosipho affectussp. nov., with type strain ik275mar^T( = DSM 23112^T = VKM B-2574^T).

The Yessentuki mineral water deposit (YMWD) is a well-known source of balneologically valuable drinking mineral water, but it has rarely been investigated in terms of the microbes inhabiting it. In this work, we have studied the microbial communities of the continuously operating production well 9, penetrating the Lower Cretaceous aquifer of the YMWD, and characterized, in detail, two novel representatives of class Ignavibacteria (Bacteroidota). One representative of the so-called XYB12-FULL-38-5 group within the family Melioribacteraceae has been isolated in pure culture, designated strain 09-Me, and physiologically characterized. It is a facultatively anaerobic thermotolerant microorganism capable of fermentation and respiration on simple and complex sugars (lichenan, xanthan gum, glucomannan, curdlan, pachyman). In addition to oxygen, ferric iron, arsenate, and elemental sulfur were also used as electron acceptors. Phylogenomic and physiological analyses reveal this novel isolate to represent a novel genus and species for which the name Stygiobacter electus gen. nov., sp. nov. is proposed. The second representative of the family Melioribacteraceae described here belonged to the so-called DSXH01 group, which comprises the dominant group (up to 28%) of the microbial community of well 9 water. The organism was characterized through the analysis of its genome, assembled from metagenome of well 9 (Ess09-04 MAG). Genes encoding enzymes of carbohydrate utilization and genes responsible for aerobic and anaerobic respiration have been identified in the genomes of both bacteria. The investigation of the environmental distribution of Stygiobacter genus-related bacteria and representatives of the lineage DSXH01 has shown that they all are typical inhabitants of the subsurface biosphere, and are often found in bioreactors. These data significantly expand our knowledge on the microbes of subsurface water basins and pave the way for future studies of the novel members of Ignavibacteria class.