c-Type Cytochrome-Dependent Formation of U(IV) Nanoparticles by Shewanella oneidensis

Authors

Matthew J. Marshall, Pacific Northwest National Laboratory
Alexander Beliaev, Pacific Northwest National LaboratoryFollow
Alice Dohnalkova, Pacific Northwest National LaboratoryFollow
David Kennedy, Pacific Northwest National LaboratoryFollow
Liang Shi, Pacific Northwest National LaboratoryFollow
Zheming Wang, Pacific Northwest National LaboratoryFollow
Maxim Boyanov, Argonne National Laboratory
Barry Lai, Argonne National Laboratory
Kenneth M. Kemner, Argonne National Laboratory
Jeffrey McLean, Pacific Northwest National Laboratory
Samantha Reed, Pacific Northwest National Laboratory
David Culley, Pacific Northwest National Laboratory
Vanessa L. Bailey, Pacific Northwest National Laboratory
Cody Simonson, Pacific Northwest National Laboratory
Daad A. Saffarini, University of Wisconsin
Margaret Romine, Pacific Northwest National Laboratory
John M. Zachara, Pacific Northwest National LaboratoryFollow
James K. Fredrickson, Pacific Northwest National LaboratoryFollow

Date of this Version

2006

Citation

PLoS Biology, August 2006 | Volume 4 | Issue 8 | e268; DOI: 10.1371/journal.pbio.0040268

Abstract

Modern approaches for bioremediation of radionuclide contaminated environments are based on the ability of microorganisms to effectively catalyze changes in the oxidation states of metals that in turn influence their solubility. Although microbial metal reduction has been identified as an effective means for immobilizing highly-soluble uranium(VI) complexes in situ, the biomolecular mechanisms of U(VI) reduction are not well understood. Here, we show that c-type cytochromes of a dissimilatory metal-reducing bacterium, Shewanella oneidensis MR-1, are essential for the reduction of U(VI) and formation of extracelluar UO₂ nanoparticles. In particular, the outer membrane (OM) decaheme cytochrome MtrC (metal reduction), previously implicated in Mn(IV) and Fe(III) reduction, directly transferred electrons to U(VI). Additionally, deletions of mtrC and/or omcA significantly affected the in vivo U(VI) reduction rate relative to wild-type MR-1. Similar to the wild-type, the mutants accumulated UO₂ nanoparticles extracellularly to high densities in association with an extracellular polymeric substance (EPS). In wild-type cells, this UO₂-EPS matrix exhibited glycocalyx-like properties and contained multiple elements of the OM, polysaccharide, and heme-containing proteins. Using a novel combination of methods including synchrotron-based X-ray fluorescence microscopy and high-resolution immune-electron microscopy, we demonstrate a close association of the extracellular UO₂ nanoparticles with MtrC and OmcA (outer membrane cytochrome). This is the first study to our knowledge to directly localize the OM-associated cytochromes with EPS, which contains biogenic UO₂ nanoparticles. In the environment, such association of UO₂ nanoparticles with biopolymers may exert a strong influence on subsequent behavior including susceptibility to oxidation by O₂ or transport in soils and sediments.