Escherichia call 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. coil by H-2 and formate. was either inhibited or repressed by 10 mM nitrate. By contrast, Tc(VII) reduction catalyzed by D. desulfuricans was less sensitive ttl nitrate when formate was the electron donor, and unaffected by 10 mM or 100 mM nitrate when H-2 was the electron donor. The optimum pH for Tc(VII) reduction by both organisms was 5.5 and the optimum temperature was 40 degrees C and 20 degrees C for E. coli and D. desulfuricans, respectively. Both strains had an apparent K-m 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 H-2. The greater bioprocess potential of D. desulfuricans was shown also by the K-s 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 H-2) were 12.5 and 800 mu mol Tc(VII) reduced/g biomass/h, but the use of an E, coli HycA muta nt (which upregulates FHL activities by approx. 50%) had a similarly enhancing effect on the rate of Tc reduction. The mom 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 flaw residence times sustaining steady-state removal of 80% of the radionuclide were 24.3 h for the wild-type E. call, 4.25 h for the upregulated mutant, and 1.5 h for D, desulfuricans.