Rectinema subterraneum sp. nov, a chemotrophic spirochaete isolated from the deep terrestrial subsurface

A novel, obligately anaerobic bacterium (strain SURF-ANA1T) was isolated from deep continental subsurface fluids at a depth of 1500 m below surface in the former Homestake Gold Mine (now Sanford Underground Research Facility, in Lead, South Dakota, USA). Cells of strain SURF-ANA1Twere Gram-negative, helical, non-spore-forming and were 0.25–0.55×5.0–75.0 µm with a wavelength of 0.5–0.62 µm. Strain SURF-ANA1Tgrew at 15–50 °C (optimally at 40 °C), at pH 4.8–9.0 (pH 7.2) and in 1.0–40.0 g l−1NaCl (10 g l−1NaCl). The strain grew chemoheterotrophically with hydrogen or mono-, di- and polysaccharides as electron donors. The major cellular fatty acids in order of decreasing abundance (comprising >5% of total) were 10-methyl C16:0, iso-C15:0, C18:2and C18:0dimethyl acetal (DMA) and C20:0methylene-nonadecanoic acid. Phylogenetic analysis based on the 16S rRNA gene sequence of strain SURF-ANA1Tindicated a closest relationship with the recently characterizedRectinema cohabitans(99%). Despite high sequence identity, because of its distinct physiology, morphology and fatty acid profile, strain SURF-ANA1Tis considered to represent a novel species within the genusRectinema, for which the nameRectinema subterraneumsp. nov. is proposed. To our knowledge, this is the first report of an isolate within the phylumSpirochaetesfrom the deep (>100 m) terrestrial subsurface. The GenBank/EMBL/DDBJ accession numbers for the 16S rRNA gene and genomic sequences of strain SURF-ANA1Tare KU359248 and GCF 009768935.1, respectively. The type strain ofRectinema subterraneumis SURF-ANA1T(=ATCC TSD-67=JCM 32656).