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

Using peptides as energy sources, H-2 as electron donor, thiosulfate as ele ctron acceptors, we isolated, from four ricefield soils originating from Fr ance and the Philippines, 52 strains of anaerobes, among which 18 reduced t hiosulfate but not sulfate. These 18 strains were strict proteolytic asacch arolytic anaerobes producing H2S when grown on thiosulfate + H-2. They exhi bited the same restriction fragment length polymorphism (R-FLP) profile (11 restriction enzymes tested). Partial sequencing of the 16S rDNA showed tha t they belonged to the genus Clostridium and were phylogenetically related to C subterminale. DNA-DNA hybridization of a representative strain with th e closest C subterminale strain (DSM 6970(T)) yielded a value of 68.9%. Pre vious counts of thiosulfate reducers unable to reduce sulfate (TSRnSR) in r icefield soils, their identification as Clostridium strains, and the known ubiquity of this genus in such soils indicate that TSRnSR of the genus Clos tridium may play a significant role in S cycling in some wetland soils. (C) 2001 Editions scientifiques et medicales Elsevier SAS.