Technetium does not occur naturally on Earth and it was the first element to be
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)
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
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:
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.