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Michigan State University

Superfund Research Program

Novel Mechanism of Uranium Reduction Via Microbial Nanowires

Project Leader: Gemma Reguera
Grant Number: R01ES017052
Funding Period: 2009-2011

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Summary

One promising strategy for the in situ bioremediation of radioactive contaminants is to stimulate the activity of dissimilatory metal-reducing microorganisms to reductively precipitate uranium (U) and other soluble toxic metals. The major goal of this research was to elucidate the mechanism of U reduction in Geobacter bacteria, which is critical to develop effective schemes for the in situ immobilization of U (VI) from groundwater. Despite its promise for the bioremediation of U contaminants, the biological mechanism behind this reaction had remained elusive for almost 20 years.

Results from the research support a model in which the conductive pili function as the primary mechanism for the reduction of uranium and cellular protection in Geobacter bacteria, with some small contribution of redox-active foci of the outer membrane, presumably c-cytochromes. These studies also revealed that significant areas of the outer membrane were voided of U reductase activity, suggesting that U (VI) was effectively prevented from permeating into the periplasm. These are regions of the outer membrane where the highly charged, rough (no O-antigen) lipopolysaccharide (LPS) is exposed and available to bind uranyl ions.

Because the nanowires also catalyze the reduction of U in biofilms and are the structural support to build them, the research team hypothesized that other components of the U-reductive machinery of the cell are also co-expressed with the pili in biofilms. Thus, the researchers developed a high-throughput assay to screen a library of ca. 4,000 transposon-insertion mutants for defects in biofilm formation. The studies demonstrated that biofilm formation in G. sulfurreducens is a developmental process requiring the expression of specific genes at each stage. Taken together, the genetic analyses of biofilm development in G. sulfurreducens revealed conservation of components required for biofilm formation in other organisms but also unique components required for the electroactivity of the biofilms.

By integrating in vivo and in vitro research platforms, the research team successfully elucidated the molecular basis of electron transfer along pilus nanowires from the nanoscale to the cellular level. The research outcomes are significant because they provided the fundamental mechanistic understanding of metal reduction by Geobacter spp. needed to design effective in situ bioremediation strategies for uranium and other toxic metals and for long-term stewardship of contaminated sites.

Furthermore, the research team developed hybrid nanointerfaces that allowed researchers to study individual components of the respiratory chain of Geobacter cells and elucidate their biological role in metal reduction in a controlled, clean environment free of biological noise. Findings from this work also provided the tools to predictively model and manipulate the activity of Geobacter during in situ bioremediation and to develop deployable nanodevices for efficient contaminant immobilization and removal.