Reduction and removal of heptavalent technetium from solution by Escherichia coli

Abstract

Anaerobic, but not aerobic, cultures of Escherichia coli accumulated Tc(VII) and reduced it to a black insoluble precipitate. Tc was the predominant element detected when the precipitate was analyzed by protoninduced X-ray emission. Electron microscopy in combination with energy-dispersive X-ray analysis showed that the site of Tc deposition was intracellular. It is proposed that Tc precipitation was a result of enzymatically mediated reduction of Tc(VII) to an insoluble oxide. Formate was an effective electron donor for Tc(VII) reduction which could be replaced by pyruvate, glucose, or glycerol but not by acetate, lactate, succinate, or ethanol. Mutants defective in the synthesis of the transcription factor FNR, in molybdenum cofactor (molybdopterin guanine dinucleotide [MGD]) synthesis, or in formate dehydrogenase H synthesis were all defective in Tc(VII) reduction, implicating a role for the formate hydrogenlyase complex in Tc(VII) reduction. The following observations confirmed that the hydrogenase III (Hyc) component of formate hydrogenlyase is both essential and sufficient for Tc(VII) reduction: (i) dihydrogen could replace formate as an effective electron donor for Tc(VII) reduction by wild-type bacteria and mutants defective in MGD synthesis; (ii) the inability of fnr mutants to reduce Tc(VII) can be suppressed phenotypically by growth with 250 M Ni 2؉ and formate; (iii) Tc(VII) reduction is defective in a hyc mutant; (iv) the ability to reduce Tc(VII) was repressed during anaerobic growth in the presence of nitrate, but this repression was counteracted by the addition of formate to the growth medium; (v) H 2 , but not formate, was an effective electron donor for a Sel ؊ mutant which is unable to incorporate selenocysteine into any of the three known formate dehydrogenases of E. coli. This appears to be the first report of Hyc functioning as an H 2 -oxidizing hydrogenase or as a dissimilatory metal ion reductase in enteric bacteria.The long-lived ␤-emitter technetium ( 99 Tc), a fission product of 235 uranium, is produced during the generation of nuclear power. In its most stable form, Tc(VII), typified by the pertechnetate ion (TcO 4 Ϫ ), is highly soluble and mobile in the environment (27). This factor, in combination with a long halflife (2.1 ϫ 10 5 years) and high biological availability as a sulfate analog (3), makes removal at the source necessary. From a recent study, it was concluded that Tc may be the critical radionuclide in determining the long-term impact of the nuclear fuel cycle (45).An approach to achieve the removal of Tc(VII) from aqueous solution may be to use metal-reducing microorganisms to reduce the radionuclide to an insoluble oxide (25, 27). For example, TcO, TcO 2 , and Tc 2 O 5 all form insoluble precipitates at neutral pH (20,29,41). Although there has been much speculation that bacteria may be able to reduce Tc enzymatically, few organisms have been shown conclusively to achieve this biotransformation (25).Anaerobically grown cultures of soil bacteria were shown by Henrot (15)...

Localization of enzymically enhanced heavy metal accumulation by Citrobacter sp. and metal accumulation in vitro by liposomes containing entrapped enzyme

“…Although the reductive bioprecipitation and removal of metal anions via growth-decoupled hydrogenase activity of SRB and of Escherichia coli has been documented Lloyd et al, 1997;Macaskie et al, 1996), the use of non-growing cells for removal of metals by biocrystallization via a single enzymic step has received relatively little attention. In addition to a reduced dependence on physiologically permissive conditions (the sensitivity is limited to the enzyme catalysing ligand production), the use of growth-decoupled cells produces a sludge which is compact and attractive in terms of the low organic content and high enrichment for the metal for recycling.…”

Aerobic bioreduction of nickel(II) to elemental nickel with concomitant biomineralization

“…Microbes can also reduce a wide range of other more toxic metals such as Cr(VI) (Wang 2000), Hg(II) (Lloyd and Lovley 2001), Co(III) (Gorby et al 1998), Pd (II) (Lloyd et al 1998;Yong et al 2002), Au(III) (Kashefi et al 2001), Ag(I) (Fu et al 2000), Mo(VI) (Bautista and Alexander 1972), and V(V) (Yurkova and Lyalikova 1991). The reduction of metalloids including As(V) (Macy et al 1996), Te(IV) (Rajwade and Paknikar 2003), and Se(VI) (Klonowska et al 2005), as well as radionuclides including U(VI) , Np (V) (Lloyd et al 2000), and Tc(VII) (Lloyd et al 1997) have also been reported. However, few studies have been done on the bioreduction of Ni(II), except for biosorption (Hussein et al 2004(Hussein et al , 2005Sar et al 2001).…”

Biogeochemical behaviour of anionic radionuclides in soil: Evidence for biotic interactions

“…Yet to correlate this result to a direct or an indirect effect of the microbial activity of the soil was difficult. Contrary to Se, microbially-mediated reduction of Tc(VII) was proved only in anaerobic condition [7,8]. But abiotic reduction of Tc(VII) should not be excluded [4].…”

“…These reduction reactions can be achieved abiotically by heterogeneous reactions with FeII-containing minerals for example [3,4]. But they have also been proved to be microbially mediated, resulting either from direct interaction between Se / Tc and some microorganisms [5,6,7,8] or from physico-chemical changes due to the microbial activity [4,9].…”