Microbial Links between Sulfate Reduction and Metal Retention in Uranium- and Heavy Metal-Contaminated Soil

Sulfate-reducing prokaryotes (SRP) can affect metal mobility either directly by reductive transformation of metal ions, e.g., uranium and chromium, into their insoluble forms or indirectly by formation of metal sulfides. This study evaluated in situ and biostimulated activity of SRP in groundwater influenced soils from a creek bank contaminated with heavy metals and radionuclides within the former uranium-mining district, Ronneburg (Germany). In situ activity of SRP, measured by the 35S-SO₄ ^2-radiotracer method, was restricted to reduced soil horizons with rates 142 ± 20 nmol cm^-3 day^-1. Although concentrations of heavy metals were enriched in the solid phase of the reduced horizons, porewater concentrations were low. XANES measurements demonstrated that ~80% of uranium was present as reduced uranium, but appeared to occur as a sorbed complex. Soil-based dsrAB clone libraries were dominated by sequences most closely affiliated to members of the Desulfobacterales, but also Desulfovibrionales, Syntrophobacteraceae and Clostridiales. ¹³C-acetate and ¹³C-lactate biostimulated soil microcosms were dominated by sulfate and Fe(III) reduction, which was associated with enrichment of similar SRP found using the dsrAB marker in soil and with sequences related to Geobacter. Concentrations of soluble nickel, cobalt, and occasionally zinc declined 100% during anoxic soil incubations. In contrast to other studies, soluble uranium increased in carbon-amended treatments reaching 1407 nM in solution. Our results suggest that (i) contaminated reduced soil with on-going sulfate reduction resulted in in situ metal attenuation and (ii) the fate of uranium mobility is not predictable and may lead to downstream contamination of adjacent ecosystems.