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and lake sediments. To gain an <br />understanding of possible biological <br />mechanisms contributing to this <br />transport, mobilization of solid -phase <br />arsenic was investigated in upper <br />Aberjona sediment microcosms. Microcosms <br />catalyzed rapid dissolution of arsenic <br />from iron arsenate, a solid -phase <br />surrogate for sedimentary arsenic, <br />mobilizing 20 -28% of the arsenic <br />present. Sterilization prevented this <br />transformation. Reduction of arsenate <br />to arsenite accompanied iron arsenate <br />dissolution, suggesting that reduction <br />was driving dissolution. <br />Sediment - conditioned, filter - sterilized <br />medium showed no arsenic - transforming <br />activity. A native enrichment culture <br />of sulfate-reducing bacteria possessed <br />one-fifth of the microcosm activity, <br />while strain MIT -13, a native <br />arsenate - reducing microorganism, showed <br />much greater activity, dissolving 38% <br />of the arsenic present. Furthermore, <br />strain MIT -13 mobilized arsenic from <br />presterilized, unamended upper Aberjona <br />11 <br />