Aims: To select an autotrophic arsenic(III)-oxidizing population, named CASO1, and to evaluate the performance of the selected bacteria in reactors. Methods and Results: An As(III)-containing medium without organic substrate was used to select CASO1 from a mining environment. As(III) oxidation was studied under batch and continuous conditions. The main organisms present in CASO1 were identified with molecular biology tools. CASO1 exhibited significant As(III)-oxidizing activity between pH 3 and 8. The optimum temperature was 25°C. As(III) oxidation was still observed in the presence of 1000 mg l )1 As(III). In continuous culture mode, the As(III) oxidation rate reached 160 mg l )1 h )1 . The CASO1 consortium contains at least two organisms -strain b3, which is phylogenetically close to Ralstonia picketii, and strain b6, which is related to the genus Thiomonas. The divergence in 16S rDNA sequences between b6 and the closest related organism was 5AE9%, suggesting that b6 may be a new species. Conclusions: High As(III)-oxidizing activity can be obtained without organic nutrient supply, using a bacterial population from a mining environment. Significance and Impact of the Study: The biological oxidation of arsenite by the CASO1 population is of particular interest for decontamination of arsenic-contaminated waste or groundwater.

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A novel bacterium, strain b6(T) (T=type strain), was isolated from a disused mine site by growth using arsenite [As(III)] as energy source in a simple mineral medium. Cells of strain b6(T) were rod-shaped, Gram-negative, non-sporulating and motile. Optimum growth occurred at temperatures between 20 and 30 degrees C, and at pH between 4.0 and 7.5. Strain b6(T) grew chemoautotrophically on As(III), sulphur and thiosulphate, and also heterotrophically on yeast extract and a variety of defined organic compounds. Several other Thiomonas strains, including the type species Thiomonas (Tm.) intermedia, were able to oxidize As(III), though only strain b6(T) and strain NO115 could grow using As(III) as sole energy source in the absence of any organic compound. The G+C content of the DNA of strain b6(T) was 65.1 mol %. Comparative small subunit (SSU) ribosomal RNA (rRNA) analysis indicated that strain b6(T) belongs to the genus Thiomonas in the beta-subdivision of the Proteobacteria. It was closely related to an unnamed Thiomonas strain (NO115) isolated from a Norwegian mining site, though sequence identities between strain b6(T) and characterized Thiomonas species were less than 95%. DNA-DNA hybridization between strain b6(T) and the type species of the genus Tm. intermedia showed less than 50% homology. On the basis of phylogenetic and phenotypic characteristics, strain b6(T) (DSM 16361(T), LMG 22795(T)) is proposed as the type strain of the new species Thiomonas arsenivorans, sp. nov.

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