| Summary: | Arsenic (As) is a common contaminant in soil-water systems, where it exists predominately as arsenate (AsV) or arsenite (AsIII), the latter of which is considered to be the more mobile and toxic form. The amount of arsenite or arsenate in natural water systems is influenced by geochemical conditions and the presence of As transforming microorganisms. Consequently, the goals of this study were to evaluate the effects of: (i) arsenic concentration on microbial populations responsible for As oxidation-reduction in a previously uncontaminated soil, and (ii) phosphate:arsenic ratio on the oxidation or reduction of arsenic. Laboratory column experiments were conducted to evaluate the influence of soil arsenic concentration on microbial community composition and to identify microorganisms and mechanisms responsible for As transformations occurring under aerobic conditions. Indigenous microorganisms within a previously uncontaminated agricultural soil were exposed to arsenite or arsenate at three concentrations (2, 20 and 200 mg As L-1) for approximately 30 days. Near complete biotic oxidation of arsenite (>96%) was observed in 2 and 20 mg As L-1 treatment columns. Results indicated that the net transformation in this soil was arsenite-oxidation; however, the addition of 200 mg arsenite L-1 inhibited oxidation. Sixty-two microorganisms were isolated from the columns; however, 43 of these were arsenate-reducers, and only 1 organism was capable of arsenite oxidation. Results of this study suggest that As perturbation of a previously uncontaminated soil does not significantly decrease microbial diversity and that cultivation techniques may be biased toward arsenate-reducing microorganisms. Phosphate is a chemical analog to arsenate and may inhibit microbial uptake of arsenate, thus preventing its reduction to arsenite. Five selected microorganisms isolated from As-treated soil columns or from As-impacted soils near Anaconda, MT were used to evaluate the effects of phosphate on arsenate-reduction and arsenite-oxidation. Cultures were initially spiked with various P:As ratios, incubated for approximately 48 hours, and analyzed periodically for arsenate and arsenite. Arsenate reduction was inhibited at high P:As ratios, but only at elevated levels of phosphate (500 and 1000 μM). This work supports that land application of phosphate could minimize microbiological reduction of arsenate to arsenite, thus reducing As bioavailability.
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