Mechanism of Uranium mineralization by Saccharomyces cerevisiae

We examined mechanism of the uranium mineralization by the yeast (Saccharomyces cerevisiae, X-2180-1B). Uptake experiments of U by S. cerevisiae were carried out for 4.2x10^-4 M U solutions with their final pHs between 3 and 5. Concentrations of U and P were measured at the predetermined intervals. After 96 hours of contact time, the U accumulated S. cerevisiae was observed by scanning electron microscope equipped with energy dispersive spectroscopy (SEM-EDS). The Kd, defined by the ratio of U accumulated in S. cerevisiae to that in solution, shows monotone increase with time below pH 4, and reached steady state within 60 hours above pH 4. During U accumulation P was released from S. cerevisiae, while no release of P was observed in the absence of U. SEM-EDS analyses of the U accumulated S. cerevisiae indicated that uranyl phosphate minerals were occurred on the surface of S. cerevisiae at any pH. Thermodynamic calculation indicated that chemical compositions of the solutions at pH above 4 were super-saturated with respect to H-autunite. On the contrary, the chemical compositions of the solutions of pHs below 4 were under-saturated with respect to H-autunite, and were super-saturated at one-order higher concentration of U(VI) or P than observed. These results indicate that U mineralization is metabolism dependent, induced by the release of P from S. cerevisiae. Two different processes are relevant to U mineralization by S. cerevisiae. One is the process in which U reacts in the solution with P released from S. cerevisiae, where the chemical composition in the solution is super-saturated with respect to H-autunite. The other is the process in which U reacts with P on the surface of S. cerevisiae, where H-autunite is occurred by local-saturation condition.