Two extremely thermophilic archaea, designated W-12 and P-4, were isolated from a geothermal vent in the tidal zone of Whale Island, New Zealand, and from geothermally heated bottom deposits of the Bay of Plenty, New Zealand, respectively. Cells of isolate W-12 are irregular cocci, 0·3--1·2 pm in diameter, motile with polar flagella. The cell envelope consists of one layer of subunits with a major protein of 75000. Cells produce protrusions of different kinds: prostheca-like, chains of bubbles, or network of fimbriae. Cells of isolate P-4 are regular cocci, 0·7--1·0 μm in diameter, motile with polar flagella. The cell envelope consists of two layers of subunits; its major protein has an of 56000. Both organisms are obligate anaerobes, fermenting peptides in the case of strain W-12, or peptides and starch in the case of P-4. Elemental sulfur is required for growth and is reduced to hydrogen sulfide. The optimal growth temperature of the new isolates is in the range 80-88 °C. The optimal growth pH is 6·5--7·2. The G+C content of the DNA of strain W-12 is 50·6 mol%, and of strain P-4 is 53·3 mol%. Based on physiological characteristics, 16S rDNA sequence comparison and DNA base composition, the new isolates were considered to be members of the genus The low level of DNA-DNA hybridization with the type strains of other species confirms the novel species status of the new isolates. The new isolates are described as sp. nov., with type strain W-12 (= DSM 10395), and sp. nov., with type strain P-4 (= DSM 10394).


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