Hydrogenase‐ and outer membrane c ‐type cytochrome‐facilitated reduction of technetium(VII) by Shewanella oneidensis MR‐1

Abstract

Summary Pertechnetate, 99 Tc(VII)O 4 – , is a highly mobile radionuclide contaminant at US Department of Energy sites that can be enzymatically reduced by a range of anaerobic and facultatively anaerobic microorganisms, including Shewanella oneidensis MR‐1, to poorly soluble Tc(IV)O 2(s) . In other microorganisms, Tc(VII)O 4 – reduction is generally considered to be catalysed by hydrogenase. Here, we provide evidence that although the NiFe hydrogenase of MR‐1 was involved in the H 2 ‐driven reduction of Tc(VII)O 4 – [presumably through a direct coupling of H 2 oxidation and Tc(VII) reduction], the deletion of both hydrogenase genes did not completely eliminate the ability of MR‐1 to reduce Tc(VII). With lactate as the electron donor, mutants lacking the outer membrane c ‐type cytochromes MtrC and OmcA or the proteins required for the maturation of c ‐type cytochromes were defective in reducing Tc(VII) to nanoparticulate TcO 2 ·nH 2 O (s) relative to MR‐1 or a NiFe hydrogenase mutant. In addition, reduced MtrC and OmcA were oxidized by Tc(VII)O 4 – , confirming the capacity for direct electron transfer from these OMCs to TcO 4 – . c ‐Type cytochrome‐catalysed Tc(VII) reduction could be a potentially important mechanism in environments where organic electron donor concentrations are sufficient to allow this reaction to dominate.

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