Inhibition of microbial arsenate reduction by phosphate

The ratio of arsenite (AsIII) to arsenate (AsV) in soils and natural waters is often controlled by the activity of As-transforming microorganisms. Phosphate is a chemical analog to AsV and, consequently, may competitively inhibit microbial uptake and enzymatic binding of AsV, thus preventing its reduction to the more toxic, mobile, and bioavailable form – AsIII. Five As-transforming bacteria isolated either from As-treated soil columns or from As-impacted soils were used to evaluate the effects of phosphate on AsV reduction and AsIII oxidation. Cultures were initially spiked with various P:As ratios, incubated for approximately 48 h, and analyzed periodically for AsV and AsIII concentration. Arsenate reduction was inhibited at high P:As ratios and completely suppressed at elevated levels of phosphate (500 and 1000 μM; P inhibition constant (Ki) ∼ 20–100 μM). While high P:As ratios effectively shut down microbial AsV reduction, the expression of the arsenate reductase gene (arsC) was not inhibited under these conditions in the AsV-reducing isolate, Agrobacterium tumefaciens str. 5B. Further, high phosphate ameliorated AsV-induced cell growth inhibition caused by high (1 mM) As pressure. These results indicate that phosphate may inhibit AsV reduction by impeding AsV uptake by the cell via phosphate transport systems or by competitively binding to the active site of ArsC.