A strictly anaerobic, thermophilic, fatty acids-degrading, sporulating sulfate-reducing bacterium was isolated from geothermal ground water. The organism stained Gram-negative and formed gas vacuoles during sporulation. Lactate, ethanol, fructose and saturated fatty acids up to C18 served as electron donors and carbon sources with sulfate as external electron acceptor. Benzoate was not used. Stoichiometric measurements revealed a complete oxidation of part of butyrate although growth with acetate as only electron donor was not observed. The rest of butyrate was oxidized to acetate. The strain grew chemolithoautotrophically with hydrogen plus sulfate as energy source and carbon dioxide as carbon source without requirement of additional organic carbon like acetate. The strain contained a c-type cytochrome and presumably a sulfite reductase P582. Optimum temperature, pH and NaCl concentration for growth were 54 degrees C, pH 7.3-7.5 and 25 to 35 g NaCl/l. The G + C content of DNA was 50.4 mol %. Strain BSD is proposed as a new species of the spore-forming sulfate-reducing genus Desulfotomaculum, D. geothermicum.
MATERIALS AND METHODS Site of sampling. To have access to deep-subsurface samples, we used the Underground Research Facility, a mine gallery located at a depth of 224 m in the Boom clay formation near Mol, Belgium. The geological cross section is as follows: 0 to 188 m deep, Neogene sands; 188 to 280 m, Boom clay; and 280 to 450 m, alternating sand and clay formations. The Boom clay, dating from the Rupelian period (30 to 35 million years ago), is dominated by illite-smectite of marine origin; 50 to 60% of the material is smaller than 2 m in diameter, and 40 to 45% is between 2 and 60 m; water content is ϳ20% (wet weight); the median porosity is between 10 and 20 nm, and the largest pore size does not exceed 0.1 to 0.2 m in diameter (11a); total organic content is ϳ3% (dry weight); the pH is 8.2 to 8.8; and the sediment temperature is approximately 20ЊC. The interstitial clay water is more than ϳ35,000 years old, consistent with its very low hydraulic conductivity (, ϳ10 Ϫ12 m s Ϫ1) (5). Sampling conditions. Clay samples have been collected at 0
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