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Abstract
The biological reduction of soluble U(VI) to the less soluble U(IV) has been proposed as a strategy to remediate uranium-contaminated sites. However, the majority of the contaminated sites contain, in addition to U(VI), competing electron acceptors (CEAs) that can either enhance or inhibit U(VI) reduction. Desulfotomaculum reducens MI-1 is a sulfate-reducing bacterium able to reduce a variety of electron acceptors including U(VI). We characterized U(VI) reduction by D. reducens in the presence of pyruvate and three CEAs: sulfate, nitrate or soluble ferric iron. In the presence of sulfate or ferric iron and U(VI), cell growth was driven by respiration of the CEA. Nitrate was not used as an electron acceptor for growth and vegetative cells grew instead by fermenting pyruvate. Sulfate remaining after sulfate reduction has ceased or the presence of nitrate did not affect U(VI) reduction. However, in the case of sulfate, the addition of H2 after the depletion of pyruvate greatly enhanced U(VI) reduction. Contrary to sulfate and nitrate, the presence of Fe(II), the product of Fe(III) reduction, abolished U(VI) reduction. The results from this investigation suggest that this microorganism and others with similar characteristics may play a role in U(VI) bioremediation efforts but only after the soluble Fe(II) produced by Fe(III) reduction has been advected away.
This work was funded by the Swiss National Foundation Division III through project 33100A0-112337 and through the US Department of Energy grant number DE-FG02-06ER64227. We thank Brad Tebo and Anna Obraztsova for providing us with the D. reducens isolate, Felippe de Alencastro and the IIE Central Environmental Laboratory for their great analytical support, Martin Schroth at ETHZ for access to the IC, Subrahmanyam Challapalli at EPFL for help in measuring H 2 , and Daniel Alessi for thermodynamic calculations of the nature of the Fe(II) precipitate. We are especially grateful to Dorothy Parker for her thorough and constructive comments on the manuscript. Support for this project was also provided by BER-ERSD project number SCW0041. Portions of this research were carried out at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences.