Microbial Communities Associated with the Oxidation of Iron and Technetium in Bioreduced Sediments

Abstract

Anoxic Tc(IV)-containing sediments representative of the UK Sellafield reprocessing facility were exposed to either air or NO₃ ⁻ to investigate redox cycling of technetium and iron. With air, oxidation of Fe(II) in the reduced sediments was accompanied by ∼75% mobilization of Tc to solution, as soluble Tc(VII). Nitrate additions resulted in the bio-oxidation of Fe(II), coupled to microbially mediated NO⁻ ₃ reduction but was accompanied by only very limited (<5%) mobilization of the reduced, sediment-bound Tc, which remained as Tc(IV). PCR-based 16S rRNA and narG gene analyzes were used to investigate changes in the microbial community during sediment oxidation by air and nitrate. Contrasting microbial communities developed in the different treatments and were dominated by Betaproteobacteria (including Herbaspirillum and Janthinobacterium spp.) in the presence of high NO⁻ ₃ concentrations. This suggests that the Betaproteobacteria are involved in the redox cycling of Fe and N in these systems, but are unable to mediate NO₃ ⁻-dependent Tc(IV) oxidation. These microorganisms may play a previously unrecognized yet pivotal role in influencing contaminant fate and transport in these environments which can have implications to the long-term stewardship of radionuclide-contaminated sediments.

ACKNOWLEDGMENTS

We thank Rob Mortimer, Lesley Neve, James Begg, and Dave Hatfield (University of Leeds), Bob Bilsborrow (SRS Daresbury), and Joyce M^cBeth (University of Manchester) for help in data acquisition. This work was supported by the UK Natural Environment Research Council (NERC) grants (NE/D00473X/1 and NE/D005361/1) and by a STFC beamtime award at SRS Daresbury.

Current affiliation for A. Geissler: Helmholtz Zentrum Dresden-Rossendorf, Dresden, Germany.