Microbial mobilization of arsenic from sediments of the Aberjona Watershed

Arsenic mobilization from aquatic sediments is an issue of concern, as water-borne arsenic can migrate into pristine areas, endangering aquatic organisms and people. Such mobilization in the Aberjona Watershed has distributed nearly 20 t of arsenic throughout river and lake sediments. To gain an understanding of possible biological mechanisms contributing to this transport, mobilization of solid-phase arsenic was investigated in upper Aberjona sediment microcosms. Microcosms catalyzed rapid dissolution of arsenic from iron arsenate, a solid-phase surrogate for sedimentary arsenic, mobilizing 20-28% of the arsenic present. Sterilization prevented this transformation. Reduction of arsenate to arsenite accompanied iron arsenate dissolution, suggesting that reduction was driving dissolution. Sediment-conditioned, filter-sterilized medium showed no arsenic-transforming activity. A native enrichment culture of sulfate-reducing bacteria possessed one-fifth of the microcosm activity, while strain MIT-13, a native arsenate-reducing microorganism, showed much greater activity, dissolving 38% of the arsenic present. Furthermore, strain MIT-13 mobilized arsenic from presterilized, unamended upper Aberjona sediments. These observations indicate that a direct microbial arsenic-mobilizing activity exists in the sediments, show that strain MIT-13 is a strong arsenic-transforming a gent native to the sediments, a nd suggest th at dissimilatory arsenic reduction may contribute to arsenic flux from anoxic sediments in the most arsenic-contaminated region of the Aberjona Watershed.