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Microbial populations responsible for the oxidation and reduction of As were examined in unsaturated (aerobic) soil columns treated with 75 μM arsenite [As(III)] or 250 μM arsenate [As(V)]. Arsenite [As(III)] was rapidly oxidized to As(V) via microbial activity, whereas no apparent reduction of As(V) was observed in the column experiments. Eight aerobic heterotrophic bacteria with varying As redox phenotypes were isolated from the same columns. Three isolates, identified as Agrobacterium tumefaciens-, Pseudomonas fluorescens-, and Variovorax paradoxus-like organisms (based on 16S sequence), were As(III) oxidizers, and all were detected in community DNA fingerprints generated by PCR coupled with denaturing gradient gel electrophoresis. The five other isolates were identified (16S gene sequence) as A. tumefaciens, Flavobacterium sp., Microbacterium sp., and two Arthrobactersp.-like organisms and were shown to rapidly reduce As(V) under aerobic conditions. Although the two A. tumefaciens-like isolates exhibited opposite As redox activity, their l6s rDNA sequences (∼1400 bp) were 100% identical, and both were shown to contain putative arsC genes. Our results support the hypothesis that bacteria capable of either oxidizing As(III) or reducing As(V) coexist and are ubiquitous in soil environments, suggesting that the relative abundance and metabolic activity of specific microbial populations plays an important role in the speciation of inorganic As in soil pore waters. [ABSTRACT FROM AUTHOR]

Investigates the microbial population associated with the reduction and enhanced mobilization of arsenic (As) in mine tailings. Relationship among pH, redox potential and mobilization of As; Methods for immobilizing trace metals and plant establishment; Enzymes involved in the detoxification pathway.