The deep subsurface is a harsh environment for life due to extreme conditions such as high temperature, high pressure, and limited availability of favorable carbon sources. Nevertheless, many microorganisms are known to thrive in such extreme environments (Magnabosco et al., 2018). Indeed, microorganisms inhabiting the deep subsurface account for 12 ∼ 20 % of the total biomass of microorganisms on Earth. However, the physiological and ecological functions of deep subsurface microorganisms remain unclear due to the lack of axenic cultures (Rinke et al., 2013).

The phylum Atribacterota is one of the dominant taxa in the subsurface environment (Gittel et al., 2009). This phylum (formerly called candidate phylum OP9) was first discovered in 1998 by the 16S rRNA gene clone library analysis in the hot spring (Obsidian Pool) of Yellowstone National Park (Hugenholtz et al., 1998). Since then, the Atribacterota members have been observed in various anaerobic environments such as geothermal systems, petroleum reservoirs, and marine sediments, and have been metagenomically characterized as their saccharolytic lifestyle: it would be grown using sugars via the Embden-Meyerhof glycolysis pathway and produce hydrogen and acetate that can be utilized by other deep subsurface microorganisms (Dodsworth et al., 2013, Nobu et al., 2016).

To date, the first isolate of the phylum Atribacterota, Atribacter laminatus (strain RT761T NBRC112890 = DSM9293), was obtained and validly described (Katayama et al., 2020). The strain RT761T is an anaerobic and meso-thermophilic sugar-fermenting bacterium that primarily produces hydrogen. Since this strain is very sensitive to hydrogen, its growth was significantly enhanced when co-cultured with a hydrogen-scavenging methanogen, suggesting the syntrophic interaction with methanogenic archaea. Intriguingly, A. laminatus possesses a unique membrane structure that is the intracellular localization of genomic DNA surrounded by a cytoplasmic inner membrane. Since only one species of the phylum Atribacterota has been described, the ecophysiology of members of this phylum is not fully understood (e.g., it is uncertain whether the above features are conserved within this phylum or the class Atribacteria).

In this study, we successfully isolated strain M15T, a novel species of the phylum Atribacterota from a high temperature deep subsurface gas field, Japan. We characterized its morphological, physiological, and phylogenetic features, revealing that the strain forms a novel family-level lineage within the order Atribacterales. Additionally, strain M15T is a thermophilic and saccharolytic bacterium with a membrane-bounded nucleoid. Based on the polyphasic analyses, we propose a new family, new genus, and new species for this strain within the class Atribacteria.