SUMMARY: Microbial sulphate reduction was examined in thermal waters, sediments and decomposing algal-bacterial mats associated with volcanic activity in Yellowstone National Park. radioactive tracer studies which demonstrated biological [S]sulphkie production from [S]sulphate at temperatures higher than 50 °C but less than 85 °C, correlated with the presence of a unique sulphate-reducing bacterium. This new species proliferated at temperatures above 45 °C but below 85 °C, and had an optimum growth temperature of 70 °C. The organism was a small Gram-negative, straight rod which displayed an outer-wall membranous layer in thin sections. This obligate anaerobe utilized pyruvate, lactate or H as electron donors and sulphate or thiosulphate as electron acceptors for growth and sulphide formation. Pyruvate alone was fermented during growth to hydrogen, acetic acid and CO. Cell extracts contained cytochrome but lacked a desulphoviridin-type bisulphite reductase. The DNA guanosine plus cytosine content was 34.4 ±1.0 mol%. Other unusual biochemical features of this extreme thermophile are discussed. Strain YSRA-1 is described as the type strain of the new genus and species


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