Monday, November 13, 2006 - 2:45 PM
124-7
Inhibition of Microbial Arsenate Reduction by Phosphate.
The relative abundance of arsenite and arsenate in soil environments is influenced by geochemical conditions and microbial transformations. Arsenate is often the predominant valence state in oxidized environments, but microbial reduction to arsenite is an important factor influencing the fate and toxicity of arsenic. Due to structural similarities, phosphate and arsenate are chemical analogs, and behave similarly in chemical and biological reactions. Consequently, high concentrations of phosphate relative to arsenate may inhibit microbial uptake of arsenate, preventing the reduction to arsenite via the ars operon. The goal of the present work was to evaluate the effects of phosphate on the microbial oxidation of arsenite or reduction of arsenate. Microorganisms used in this study included two arsenite-oxidizing microorganisms (Agrobacterium tumefaciens str. 5A and Variovorax sp. str. RM1), and three arsenate-reducing organisms (Agrobacterium tumefaciens str. 5B, Arthrobacter sp. str. DM2, and Bacillus sp. str. DM3), all of which were isolated from soils with long-term As-contamination. Liquid cultures were spiked with various As:PO4 ratios and grown in a synthetic soil solution medium at 30 OC for approximately 48 hours on a shaker (120 rpm). Arsenic:PO4 ratios ranging from 20:1 to 0.1:1 were achieved using varying concentrations of arsenite, arsenate and phosphate. Results show that increased phosphate had no effect on arsenite oxidation by either the Variovorax or A. tumefaciens 5A strains. Conversely, phosphate levels above 0.5 mM resulted in a significant decrease in the amount of arsenate reduced in all three of the arsenate-reducing isolates. The inhibition of arsenate reduction is more pronounced at higher concentrations of phosphate, even while arsenate: phosphate ratios remain constant. These results demonstrate the importance of phosphate on the microbial reduction of arsenate, and suggest that phosphate concentrations in soils or natural waters may be an important parameter indirectly limiting the formation of arsenite via microbial processes.
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