Release of arsenic from soil by a novel dissimilatory arsenate-reducing bacterium, Anaeromyxobacter sp. strain PSR-1

Abstract

A novel arsenate-reducing bacterium, designated strain PSR-1, was isolated from arsenic-contaminated soil. Strain PSR-1 was phylogenetically closely related to Anaeromyxobacter dehalogenans 2CP-1(T) with 16S rRNA gene similarity of 99.7% and coupled the oxidation of acetate with the reduction of arsenate. Arsenate reduction was inhibited almost completely by respiratory inhibitors such as dicumarol and 2-heptyl-4-hydroxyquinoline N-oxide. Strain PSR-1 also utilized soluble Fe(III), ferrihydrite, nitrate, oxygen, and fumarate as electron acceptors. Strain PSR-1 catalyzed the release of arsenic from arsenate-adsorbed ferrihydrite. In addition, inoculation of washed cells of strain PSR-1 into sterilized soil successfully reproduced arsenic release. Arsenic K-edge X-ray absorption near-edge structure (XANES) analysis revealed that the proportion of arsenite in the soil solid phase actually increased from 20% to 50% during incubation with washed cells of strain PSR-1. These results suggest that strain PSR-1 is capable of reducing not only dissolved arsenate but also arsenate adsorbed on the soil mineral phase. Arsenate reduction by strain PSR-1 expands the metabolic versatility of Anaeromyxobacter dehalogenans. Considering its distribution throughout diverse soils and anoxic sediments, Anaeromyxobacter dehalogenans may play a role in arsenic release from these environments.

Fig 1

PCR-DGGE analysis of the arsenate-reducing enrichment prepared from arsenic-contaminated soil. The DGGE profiles of the original soil (lane 1) and the enrichment (lane 2) are shown. Arrows indicate bands recovered for DNA sequencing (see Table 1).

Fig 2

(A) Scanning electron micrograph of strain PSR-1. (B) Cells and spore- or cyst-like structure. A bleb-like structure is also found in the terminal ends of the cells. In both cases, the strain was grown using acetate as the electron donor and arsenate as the electron acceptor.

Fig 3

Phylogenetic tree showing the relationship between strain PSR-1 and related Myxococcales species within the Deltaproteobacteria on the basis of 16S rRNA gene sequences. The tree was constructed using the neighbor-joining method. Numbers at nodes show bootstrap values obtained from 1,000 resamplings, but bootstrap values below 500 were omitted. The GenBank accession number for each reference strain is shown in parentheses. The scale bar indicates 1% estimated sequence divergence.

Fig 4

Growth of strain PSR-1 on arsenate as the sole electron acceptor. The strain was grown on 2 mM acetate as the electron donor and 5 mM arsenate as the electron acceptor. A representative result from 3 independent experiments is shown.

Fig 5

Bacterial dissolution of arsenic from arsenate-adsorbed ferrihydrite. The minimal medium containing arsenate-adsorbed ferrihydrite as the sole electron acceptor was prepared and was inoculated with washed cells of Shewanella oneidensis MR-1 (A) and with washed cells of strain PSR-1 pregrown on arsenate (B) or on Fe(III)-NTA (C). Symbols represent the mean values obtained for triplicate determinations, and bars indicate standard deviations.

Fig 6

Arsenic K-edge XANES spectra of reference materials and sterile paddy soil incubated with washed cells of strain PSR-1. The result in which cells grown on Fe(III)-NTA were inoculated is shown. A similar result was obtained when cells grown on arsenate were inoculated. Dotted lines represent the linear combination of XANES spectra from reference compounds to reproduce the experimental spectra.