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 5·9%, 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.

Background: Thiomonasstrains are ubiquitous in arsenic-contaminated environments. Differences betweenThiomonasstrains in the way they have adapted and respond to arsenic have never been studied in detail. For this purpose, fiveThiomonasstrains, that are interesting in terms of arsenic metabolism were selected:T. arsenivorans,Thiomonasspp. WJ68 and 3As are able to oxidise As(III), whileThiomonassp. Ynys1 andT. perometabolisare not. Moreover,T. arsenivoransand 3As present interesting physiological traits, in particular that these strains are able to use As(III) as an electron donor.Results: The metabolism of carbon and arsenic was compared in the fiveThiomonasstrains belonging to two distinct phylogenetic groups. Greater physiological differences were found between these strains than might have been suggested by 16S rRNA/rpoAgene phylogeny, especially regarding arsenic metabolism. Physiologically,T. perometabolisand Ynys1 were unable to oxidise As(III) and were less arsenic-resistant than the other strains. Genetically, they appeared to lack theaoxarsenic-oxidising genes and carried only a singlearsarsenic resistance operon.Thiomonas arsenivoransbelonged to a distinct phylogenetic group and increased its autotrophic metabolism when arsenic concentration increased. Differential proteomic analysis revealed that inT. arsenivorans, therbc/cbbgenes involved in the assimilation of inorganic carbon were induced in the presence of arsenic, whereas these genes were repressed inThiomonassp. 3As.Conclusion: Taken together, these results show that these closely related bacteria differ substantially in their response to arsenic, amongst other factors, and suggest different relationships between carbon assimilation and arsenic metabolism.