Growth of <i>Desulfovibrio vulgaris</i> When Respiring U(VI) and Characterization of Biogenic Uraninite

Abstract

The capacity of <i>Desulfovibrio vulgaris</i> 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 <i>D. vulgaris</i> at a low initial biomass (10 to 20 mg/L of protein) that could reveal biomass growth. <i>D. vulgaris</i> 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