sp. nov., an arginine biosynthesis archaeal species isolated from the Central Indian Ocean ridge.

A strictly anaerobic hyperthermophilic archaeon, designated strain IOH2, was isolated from a deep-sea hydrothermal vent (Onnuri vent field) area on the Central Indian Ocean Ridge. Strain IOH2 showed high 16S rRNA gene sequence similarity to MM 739 (99.42 %), DSM 10322 (99.28 %), P5 (99.21 %), DSM 5473 (99.13 %), ' T7324 (99.13 %), TY (98.92 %) and Bio-pl-0405IT2 (98.01 %), with all other strains showing lower than 98 % similarity. The average nucleotide identity and DNA-DNA hybridization values were highest between strain IOH2 and MM 739 (79.33 and 15.00 %, respectively); these values are much lower than the species delineation cut-offs. Cells of strain IOH2 were coccoid, 1.0-1.2 µm in diameter and had no flagella. Growth ranges were 60-85 °C (optimum at 80 °C), pH 4.5-8.5 (optimum at pH 6.3) and 2.0-6.0 % (optimum at 4.0 %) NaCl. Growth of strain IOH2 was enhanced by starch, glucose, maltodextrin and pyruvate as a carbon source, and elemental sulphur as an electron acceptor. Through genome analysis of strain IOH2, arginine biosynthesis related genes were predicted, and growth of strain IOH2 without arginine was confirmed. The genome of strain IOH2 was assembled as a circular chromosome of 1 946 249 bp and predicted 2096 genes. The DNA G+C content was 39.44 mol%. Based on the results of physiological and phylogenetic analyses, sp. nov. is proposed with type strain IOH2 (=MCCC 4K00089=KCTC 25190).