Reductive precipitation of the nuclear fuel cycle contaminant Tc(VII) by Geobacter sulfurreducens

echnetium is a key radioactive pollutant in subsurface environments contaminated with radioactive waste. However, dissimilatory Fe(III)-reducing bacteria can control its mobility through the enzymatic reduction and precipitation of soluble Tc(VII) to insoluble Tc(IV). We have investigated the mechanism of electron transfer to Tc(VII) in the model dissimilatory Fe(III)-reducing bacterium Geobacter sulfurreducens. Reduction of Tc(VII) by G. sulfurreducens required hydrogen as the electron donor, suggesting the involvement of a hydrogenase, while CO profiling indicated that Ni and Fe are the metal cofactors. Enzymatic activity and the reduced Tc(IV) precipitate was localized in the periplasm using Cu(II) (a selective inhibitor for periplasmic hydrogenases) and transmission electron microscopy. Finally, a deletion mutant unable to synthesize the primary periplasmic NiFe hydrogenase of G. sulfurreducens was unable to reduce Tc(VII), confirming a role for this protein in electron transfer to the fission product.