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

A novel, obligately anaerobic bacterium (strain SURF-ANA1^T) 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-ANA1^T were 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-ANA1^T grew 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^-1 NaCl (10 g l^-1 NaCl). 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 C_16:0, iso-C_15:0, C_18:2 and C_18:0 dimethyl acetal (DMA) and C_20:0 methylene-nonadecanoic acid. Phylogenetic analysis based on the 16S rRNA gene sequence of strain SURF-ANA1^T indicated a closest relationship with the recently characterized Rectinema cohabitans (99%). Despite high sequence identity, because of its distinct physiology, morphology and fatty acid profile, strain SURF-ANA1^T is considered to represent a novel species within the genus Rectinema, for which the name Rectinema subterraneum sp. nov. is proposed. To our knowledge, this is the first report of an isolate within the phylum Spirochaetes from the deep (>100 m) terrestrial subsurface. The GenBank/EMBL/DDBJ accession numbers for the 16S rRNA gene and genomic sequences of strain SURF-ANA1^T are KU359248 and GCF 009768935.1, respectively. The type strain of Rectinema subterraneum is SURF-ANA1^T (=ATCC TSD-67=JCM 32656).