Toxic effects of uranium on Desulfovibrio desulfuricans G20

The toxic effects of U(VI) were studied using Desulfovibrio desulfuricans G20 in a medium containing bicarbonate or 1,4-piperazinediethane sulfonic acid disodium salt monohydrate (PIPES) buffer (each at 30 mM and pH 7). Uranium(VI) toxicity was dependent on the medium buffer and was observed in terms of longer lag times and, in some cases, no measurable growth. The minimum inhibiting concentration was 140 microM U(VI) in PIPES-buffered medium. This is 36-fold lower than that reported previously for D. desulfuricans. For all cases in which D. desulfuricans G20 grew in the presence of U(VI), the final cell protein yield was equivalent to that of the U(VI)-free control. In 24 h, D. desulfuricans G20 (total cell protein, 40 mg/L) removed 50 FiM U(VI) from solution in PIPES buffer, as compared to 96 microM U(VI) in bicarbonate buffer under anaerobic, nongrowth conditions. Even though the solubility of U(VI) was significantly lower in PIPES buffer than in bicarbonate buffer, U(VI) was much more toxic in PIPES buffer than in bicarbonate buffer. Analysis of thin sections of D. desulfuricans G20 treated with 90 microM U(VI) in medium containing PIPES buffer revealed that only a very small fraction of cells had reduced U precipitates in the periplasmic spaces. In the presence of bicarbonate buffer, however, reduced U was observed not only in the periplasm but also in the cytoplasm. Selected-area electron diffraction patterns and crystallographic analysis of transmission-electron microscopic lattice fringe images confirmed the structure of precipitated U in the cell periplasm and cytoplasm as being that of uraninite. These results suggest that U(VI) toxicity and the detoxification mechanisms of D. desulfuricans G20 depend greatly on the chemical forms of U(VI) that are present.