Dynamic gamma-camera imaging of radiotracer technetium ((99m)Tc) was used to assess the impact of biostimulation of metal-reducing bacteria on technetium mobility at 10(-12) mol L(-1) concentrations in sediments. Addition of the electron donor acetate was used to stimulate a redox profile in sediment columns, from oxic to Fe(III)-reducing conditions. When (99m)Tc was pumped through the columns, real-time gamma-camera imaging combined with geochemical analyses showed technetium was localized in regions containing biogenic Fe(II). In parallel experiments, electron microscopy with energy-dispersive X-ray (EDX) mapping confirmed sediment-bound Tc was associated with iron, while X-ray absorption spectroscopy (XAS) confirmed reduction of Tc(VII) to poorly soluble Tc(IV). Molecular analyses of microbial communities in these experiments supported a direct link between biogenic Fe(II) accumulation and Tc(VII) reductive precipitation, with Fe(III)-reducing bacteria more abundant in technetium immobilization zones. This offers a novel approach to assessing radionuclide mobility at ultratrace concentrations in real-time biogeochemical experiments, and confirms the effectiveness of biostimulation of Fe(III)-reducing bacteria in immobilizing technetium.