Technetium does not occur naturally on Earth and it was the first element to be produced artificially. ⁹⁹Tc is a β-emitting radionuclide with a long half-life (2.1 X 10⁵ years) and is generated as a byproduct of nuclear power plants and atomic weapons tests. Technetium is most stable as Tc(VII), which typically occurs as the highly soluble pertechnetate ion (TcO₄^-). Reduced technetium oxides are insoluble, and bioreduction of technetium is a promising approach to immobilizing soluble technetium in hazardous waste sites.

Direct and indirect reduction of Tc(VII), which is sometimes concomitant with technetium bioaccumulation, has been described in a variety of bacterial species (reviewed by Lloyd and Macaskie, 1997). Shewanella putrefaciens was shown to reduce Tc(VII) to Tc(V) and Tc(IV) without accumulating technetium, and Geobacter metallireducens was found to reduce Tc(VII) to Tc(V) while accumulating 70% of the technetium in solution (Lloyd and Macaskie, 1996). Technetium accumulation by Halomonas sp. Tc-202 under aerobic conditions has been reported (Fujimoto and Morita, 2006).

The enzymes and abiotic reducing agents that catalyze technetium reduction by some bacteria have been identified. Eschericia coli cells grown anaerobically were shown to reduce Tc(VII) via the hydrogenase III (Hyc) component of formate hydrogenylase, and reduced Tc(VII) was accumulated intracellularly as a black precipitate (Lloyd et al, 1997). In Desulfovibrio fructosovorans, a periplasmic nickel-iron hydrogenase functions as a Tc(VII) reductase (De Luca et al, 2001). The radiation-resistant bacterium Deinococcus radiodurans appears to indirectly reduce Tc(VII) by using the humic acid analog anthraquinone-2,6-disulfonate (AQDS) as an electron shuttle (Fredrickson et al, 2000). Cell-mediated Fe(III) reduction by Geobacter sulferreducens generated Fe(II), which subsequently reduced Tc(VII) to Tc(IV) (Lloyd et al, 2000).

For more information:

Search Medline for technetium metabolism AND bacteria

Lloyd JR, Macaskie LE. A novel phosphorImager-based technique for monitoring the microbial reduction of technetium. Appl Environ Microbiol. 1996;62:578-82.

Wildung RE, Gorby YA, Krupka KM, Hess NJ, Li SW, Plymale AE, McKinley JP, Fredrickson JK. Effect of electron donor and solution chemistry on products of dissimilatory reduction of technetium by Shewanella putrefaciens. Appl Environ Microbiol. 2000;66:2451-60.