Thermocladium modestius gen. nov., sp. nov., a new genus of rod-shaped, extremely thermophilic crenarchaeote

Three strains of novel, extremely thermophilic, rod-shaped crenarchaeotes were isolated from acidic hot spring areas in Japan. Cells of the three strains were straight or slightly curved rods and occasionally branched out singly or extensively, or had spherical bodies protruding at the ends of the cells. They were heterotrophs that grew anaerobically or microaerobically. the presence of CO, in the gas phase, archaeal cell-extracts and a vitamin mixture stimulated growth of the strains. Growth occurred at 45-82 degrees C and ph 2.6-5.9 and was optimal around 75 degrees C and ph 4.0. The strains utilized glycogen, starch, gelatin and various proteinaceous complex compounds as carbon sources. They required sulfur, thiosulfate or L-cystine as possible electron accepters. The lipids mainly consisted of various cyclic glycerol-bisdiphytanyl-glycerol tetraethers. The G+C contents of the genomic DNAs were 52 mol%. Comparison of the 16S rDNA sequences indicated that they belonged to a separate lineage in the family Thermoproteaceae. The three strains were included in a single species due to high levels of DNA-DNA hybridization values. Based upon these results, the new isolates were assigned to a new genus and species in the family Thermoproteaceae, Thermocladium modestius gen. nov., sp. nov. The type strain is Thermocladium modestius IC-125(T) (= JCM 10088(T)).