Growth of Desulfovibrio vulgaris when respiring U(VI) and characterization of biogenic uraninite

The capacity of Desulfovibrio vulgaris to reduce U(VI) was studied previously with nongrowth conditions involving a high biomass concentration; thus, bacterial growth through respiration of U(VI) was not proven. In this study, we conducted a series of batch tests on U(VI) reduction by D. vulgaris at a low initial biomass (10 to 20 mg/L of protein) that could reveal biomass growth. D. vulgaris grew with U(VI) respiration alone, as well as with simultaneous sulfate reduction. Patterns of growth kinetics and solids production were affected by sulfate and Fe²⁺. Biogenic sulfide nonenzymatically reduced 76% of the U(VI) and greatly enhanced the overall reduction rate in the absence of Fe²⁺ but was rapidly scavenged by Fe²⁺ to form FeS in the presence of Fe²⁺. Biogenic U solids were uraninite (UO₂) nanocrystallites associated with 20 mg/g biomass as protein. The crystallite thickness of UO₂ was 4 to 5 nm without Fe²⁺ but was <1.4 nm in the presence of Fe²⁺, indicating poor crystallization inhibited by adsorbed Fe²⁺ and other amorphous Fe solids, such as FeS or FeCO₃. This work fills critical gaps in understanding the metabolic utilization of U by microorganisms and formation of UO₂ solids in bioremediation sites.