Microbial reduction of technetium by Escherichia coli and Desulfovibrio desulfuricans: Enhancement via the use of high-activity strains and effect of process parameters

Escherichia coli and Desulfovibrio desulfuricans reduce Tc(VII) (TcO4−) with formate or hydrogen as electron donors. The reaction is catalyzed by the hydrogenase component of the formate hydrogenlyase complex (FHL) of E. coli and is associated with a periplasmic hydrogenase activity in D. desulfuricans. Tc(VII) reduction in E. coli by H2 and formate was either inhibited or repressed by 10 mM nitrate. By contrast, Tc(VII) reduction catalyzed by D. desulfuricans was less sensitive to nitrate when formate was the electron donor, and unaffected by 10 mM or 100 mM nitrate when H2 was the electron donor. The optimum pH for Tc(VII) reduction by both organisms was 5.5 and the optimum temperature was 40°C and 20°C for E. coli and D. desulfuricans, respectively. Both strains had an apparent Km for Tc(VII) of 0.5 mM, but Tc(VII) was removed from a solution of 300 nM TcO4− within 30 h by D. desulfuricans at the expense of H2. The greater bioprocess potential of D. desulfuricans was shown also by the Ks for formate (>25 mM and 0.5 mM for E. coli and D. desulfuricans, respectively), attributable to the more accessible, periplasmic localization of the enzyme in the latter. The relative rates of Tc(VII) reduction for E. coli and D. desulfuricans (with H2) were 12.5 and 800 μmol Tc(VII) reduced/g biomass/h, but the use of an E. coli HycA mutant (which upregulates FHL activities by approx. 50%) had a similarly enhancing effect on the rate of Tc reduction. The more rapid reduction of Tc(VII) by D. desulfuricans compared with the E. coli strains was also shown using cells immobilized in a hollow‐fiber reactor, in which the flow residence times sustaining steady‐state removal of 80% of the radionuclide were 24.3 h for the wild‐type E. coli, 4.25 h for the upregulated mutant, and 1.5 h for D. desulfuricans. © 1999 John Wiley & Sons, Inc. Biotechnol Bioeng 66: 122–130, 1999.