New insights into the behavior and biochemical mechanism of microbial Tc(VII) reduction via the investigation of electron transfer

Abstract

Technetium-99 (99Tc), a long-lived and highly mobile radionuclide, is recognized as a problematic contaminant in groundwater. Although the microbial reduction and immobilization of Tc(VII) have been substantially documented, the reduction behavior of Tc(VII) and the associated biochemical mechanisms remain unclear. This study employed the facultative Klebsiella variicola strain X-21 (K. variicola X-21) to shed further light on these intricate processes, particularly on the electron donors for Tc(VII) reduction in the oligotrophic system and the mechanism of electron transfer. The results revealed that more than 68 % of the initial Tc(VII) (1 × 10-4 M) could be reduced and deposited by K. variicola X-21 under anaerobic conditions. The presence of Fe3+ could promote the Tc(VII) reduction induced by K. variicola X-21, while NO3– and Cu2+ inhibit the reduction. The self-secreted exopolysaccharides of K. variicola X-21 can be metabolized to supply electrons for the Tc(VII) reduction in the absence of effective electron donors such as pyruvate, glycerol, formate, and glucose. The electron transfer of the Tc(VII) reduction induced by K. variicola X-21 depends on succinate-CoQ reductase (complexes II), CoQH2-cytochrome c reductase (complexes III), and cytochrome c oxidase (complexes IV) of the respiratory chain. Combined with the location and valence state of the end products, the evidence becomes definitive that the Tc(VII) reduction mainly occurs within the cytoplasm of K. variicola X-21, resulting in the intracellular accumulation of technetium in the form of amorphous Tc(IV) species.