Identification of thiosulfate- and sulfur-reducing bacteria unable to reduce sulfate in ricefield soils

Using peptides as energy sources, H2 as electron donor, thiosulfate as electron acceptors, we isolated, from four ricefield soils originating from France and the Philippines, 52 strains of anaerobes, among which 18 reduced thiosulfate but not sulfate. These 18 strains were strict proteolytic asaccharolytic anaerobes producing H2S when grown on thiosulfate + H2. They exhibited the same restriction fragment length polymorphism (RFLP) profile (11 restriction enzymes tested). Partial sequencing of the 16S rDNA showed that they belonged to the genus Clostridium and were phylogenetically related to C. subterminale. DNA–DNA hybridization of a representative strain with the closest C. subterminale strain (DSM 6970T) yielded a value of 68.9%. Previous counts of thiosulfate reducers unable to reduce sulfate (TSRnSR) in ricefield soils, their identification as Clostridium strains, and the known ubiquity of this genus in such soils indicate that TSRnSR of the genus Clostridium may play a significant role in S cycling in some wetland soils.