gen. nov., sp. nov., a thermoacidophilic, obligate ectosymbiotic archaeon, and proposal of fam. nov., ord. nov. and class. nov.

A co-culture of a novel thermoacidophilic, obligate symbiotic archaeon, designated as strain MJ1, with its specific host archaeon strain MJ1HA was obtained from a terrestrial hot spring in Japan. Strain MJ1 grew in the co-culture under aerobic conditions. Coccoid cells of strain MJ1 were 200-500 nm in diameter, and attached to the MJ1HA cells in the co-culture. The ranges and optima of the growth temperature and pH of strain MJ1 in the co-culture were 60-75 °C (optimum, 65-70 °C) and pH 1.0-4.0 (optimum, pH 2.5), respectively. Core lipids of dialkyl glycerol tetraethers (GDGT)-3 and GDGT-4 were highly abundant in MJ1 cells concentrated from the co-culture. Strain MJ1 has a small genome (0.67 Mbp) lacking genes for biosynthesis of essential biomolecules, such as nucleotides, lipids and ATP. The genomic DNA G+C content was 24.9 mol%. The 16S rRNA gene sequence of strain MJ1 was most closely related to that of the cultivated species, ' strain N7A (85.8 % similarity). Based on phylogenetic and physiological characteristics, we propose the name gen. nov., sp. nov. to accommodate the strain MJ1 (=JCM 33616=DSM 111728). In addition, we propose the names fam. nov., ord. nov., and class. nov. to accommodate the novel genus.