Spirochaeta psychrophila sp. nov., a psychrophilic spirochaete isolated from subseafloor sediment, and emended description of the genus Spirochaeta An obligately anaerobic, psychrophilic spirochaete, strain MO-SPC1 T , was isolated from a methanogenic microbial community grown in a continuous-flow bioreactor. Originally, this community was obtained from subseafloor sediments off the Shimokita Peninsula of Japan in the north-western Pacific Ocean. The cells were motile, Gram-stain-negative, helical, 0.25-0.55¾3.6-15 mm, with a wavelength of approximately 0.5-0.6 mm. Strain MO-SPC1 T grew at 0-18 6C (optimally at 15 6C), at pH 6.0-7.5 (optimally at pH 6.8-7.0) and in 20-70 g NaCl l "1 (optimally at 30-40 NaCl l "1 ). The strain grew chemo-organotrophically with mono-, di-and polysaccharides. The major end products of glucose fermentation were acetate, ethanol, hydrogen and carbon dioxide. The abundant polar lipids of strain MO-SPC1 T were diphosphatidylglycerol, phosphatidylglycerol, unknown phospholipids and an unknown lipid. The major cellular fatty acids (.5 % of the total) were C 14 : 0 , C 16 : 0 , iso-C 13 : 0 , iso-C 14 : 0 , iso-C 15 : 0 , anteiso-C 13 : 0 and anteiso-C 15 : 0 . To the best of our knowledge, this is the first report of the fatty acids iso-C 13 : 0 and anteiso-C 13 : 0 from a species of the genus Spirochaeta. Isoprenoid quinones were not found. The G+C content of the genomic DNA was 39.8 mol%. 16S rRNA gene sequence-based phylogenetic analysis showed that strain MO-SPC1 T was affiliated with the genus Spirochaeta, and its closest relatives were Spirochaeta isovalerica MA-2 T (95.6 % sequence identity) and Spirochaeta litoralis R1 T (89.4 %). Based on its phenotypic characteristics and phylogenetic traits, strain MO-SPC1 T is placed in a separate taxon at the level of a novel species within the genus Spirochaeta, for which the name Spirochaeta psychrophila sp. nov. is proposed, reflecting its true psychrophilic physiology. The type strain is MO-SPC1 T (5JCM 17280 T 5DSM 23951 T ). To our knowledge, this is the first report of an isolate of the phylum Spirochaetes from a deep-sea sedimentary environment, and of an obligately psychrophilic nature.