A thermophilic, sulfate-reducing bacterium, strain MT-96T, was isolated from an active, marine, shallow-water hydrothermal vent system. It used a large variety of substrates, ranging from simple organic compounds to long-chain fatty acids, as electron donors. Autotrophic growth was possible with H2 and CO2 in the presence of sulfate. Sulfate, thiosulfate and sulfite were used as electron acceptors. Sulfur and nitrate were not reduced. Fermentative growth was obtained with pyruvate, but not with fumarate or malate. Substrate oxidation was usually complete, leading to production of CO2, but at high substrate concentrations acetate accumulated. The oval-shaped cells were 0.8-1.0 microm in width and 1.5-2.5 microm in length. Cells were motile during the early-exponential-growth phase, but motility rapidly declined during later growth phases. Spores were not produced and cells stained Gram-negative. The temperature limits for growth were between 37 and 64 degrees C, with an optimum at 60 degrees C. Growth was observed at salinities ranging from 15 to 78 g NaCl l(-1), with optimum growth in the presence of 32-36 g NaCl l(-1). This might reflect an adaptation to the elevated salinity of the hydrothermal fluid. The G+C content of the DNA was 59.5 mol%. Vitamins or other supplements were not required. Based on the 16S rRNA gene sequence, strain MT-96T belonged in the delta-subclass of the Proteobacteria. Strain MT-96T was found to be phenotypically and phylogenetically related to Desulfacinum infernum (< 95.3% sequence identity) and represents a new member of the genus Desulfacinum. The name Desulfacinum hydrothermale is proposed for this strain; the type strain is MT-96T (= DSM 13146).