New thermoacidophilic organisms that were able to grow anaerobically on starch were isolated from the acidic hot springs of Kamchatka. Strain 1904T, isolated from a hot spring of the Moutnovski volcano, was characterized in detail. Its cells were regular or irregular cocci that were 1-2 microm in diameter, non-motile, and had a cell envelope consisting of one layer of subunits. The new organism was a hyperthermophile, growing in the temperature range 60-92 degrees C (with an optimum at 85 degrees C), an acidophile, having the pH range for growth of 2.0-6.0 (with an optimum at 3.8), and an obligate anaerobe. It fermented starch, forming acetate as the main growth product. Other growth substrates were yeast extract, beef extract and soya extract. Growth on yeast extract, beef extract and soya extract was stimulated by elemental sulfur, which was reduced to H2S. Acetate, arabinose, cellulose, formate, fructose, galactose, glucose, glycine, guar gum, lichenan, malate, maltose, methanol, pectin, pyruvate, propionate, xylan, xylose or a mixture of amino acids failed to support growth both in the presence and the absence of sulfur. When starch was used as the growth substrate, yeast extract (100 mg l(-1)) was required as a growth factor. The G+C content of the DNA was found to be 53.8 mol%. Comparison of the complete 16S rDNA sequence with databases revealed that the new isolate belonged to the kingdom Crenarchaeota. It was not closely related to any described genera (showing sequence similarity below 90.8%) and formed a separate branch of the Crenarchaeota. On the basis of physiological differences and rRNA sequence data, a new genus--Acidilobus--is proposed, the type species being Acidilobus aceticus strain 1904T (= DSM 11585T).


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