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

Eighteen strains of anaerobic thiosulfate-reducing bacteria unable to use sulfate as electron acceptor (TSRnSR) were isolated from four ricefield soils originating from France and the Philippines, using peptides as energy sources, H2 as electron donor, thiosulfate as electron acceptors, and four enrichment methods to vary the selection pressure. They were strict proteolytic asaccharolytic anaerobes producing H2S when grown on thiosulfate + H2. They exhibited the same 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 TSRnSR in ricefield soils, their identification as Clostridium strains, and the known ubiquity of this genus in such soils indicate that TSRnSR may play a significant role in S cycling in some wetland soils. (Résumé d'auteur)