Microorganisms play a significant role in the speciation and mobility of arsenic in the environment. In this study, the oxidation of arsenite [As(III)] to arsenate [As(V)] linked to chlorate (ClO_(3)~(?)) reduction was shown to be catalyzed by sludge samples, enrichment cultures (ECs), and pure cultures incubated under anaerobic conditions. No activity was observed in treatments lacking inoculum or with heat-killed sludge, or in controls lacking ClO_(3)~(?). The As(III) oxidation was linked to the complete reduction of ClO_(3)~(?) to Cl~(?), and the molar ratio of As(V) formed to ClO_(3)~(?) consumed approached the theoretical value of 3:1 assuming the e ~(?) equivalents from As(III) were used to completely reduce ClO_(3)~(?). In keeping with O_(2) as a putative intermediate of ClO_(3)~(?) reduction, the ECs could also oxidize As(III) to As(V) with O_(2) at low concentrations. Low levels of organic carbon were essential in heterotrophic ECs but not in autotrophic ECs. 16S rRNA gene clone libraries indicated that the ECs were dominated by clones of Rhodocyclaceae (including Dechloromonas , Azospira , and Azonexus phylotypes) and Stenotrophomonas under autotrophic conditions. Additional phylotypes ( Alicycliphilus , Agrobacterium , and Pseudoxanthomonas ) were identified in heterotrophic ECs. Two isolated autotrophic pure cultures, Dechloromonas sp. strain ECC1-pb1 and Azospira sp. strain ECC1-pb2, were able to grow by linking the oxidation of As(III) to As(V) with the reduction of ClO_(3)~(?). The presence of the arsenite oxidase subunit A ( aroA ) gene was demonstrated with PCR in the ECs and pure cultures. This study demonstrates that ClO_(3)~(?) is an alternative electron acceptor to support the microbial oxidation of As(III).
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