CHARACTERIZATION OF MICROBIAL SELENIUM REDUCTION IN SOUTHEASTERN IDAHO

An assessment completed in the fall of 1997 showed elevated selenium concentrations as a result of mining activities at the US Western Phosphate Resource in southeast Idaho, USA. Natural weathering processes have resulted in the leaching of Se from waste mine piles and from contaminated seleniferous shales that had been used for land reclamation. We have begun an initial characterization of the microbial Se transformations occurring within contaminated soils. Anaerobic enrichments were initiated using seleniferous shale with acetate or lactate as carbon sources and selenate as the electron acceptor. The formation of a red precipitate, elemental selenium, occurred in all enrichments and three distinct pure isolates were obtained using roll tubes. All three isolates reduced selenate, but not selenite, and nitrate as terminal electron acceptors using lactate or acetate as a carbon source. None of the isolates could grow using sulfate, iron or manganese oxides. Differences between the isolates included maximum selenate concentrations reduced, ranging from 1.5 to 8.5 mM, and optimal growth temperatures. In addition, one of the isolates used formate and all isolates used acetate, lactate, glucose and yeast extract as electron donors. Aerobic enrichments using yeast extract or casamino acids yielded isolates distinct from those obtained from the anaerobic enrichments. The results gained from these studies are the first step towards assessing the bioremediation potential of the naturally occurring microbial population.