Superfund Research Program
Novel Mechanism of Uranium Reduction Via Microbial Nanowires
Project Leader: Gemma Reguera
Grant Number: R01ES017052
Funding Period: 2009-2011
The reduction of Fe(III) oxides by Geobacter bacteria can be stimulated in situ and results in the concomitant reductive precipitation of uranium. Our team has demonstrated that the immobilization of uranium reduction, like the reduction of Fe(III) oxides, requires the expression of conductive protein filaments (pilus nanowires) by the cells. To investigate their role in uranium reduction, the team aims at:
- identifying the molecular basis of nanowire-mediated electron transfer using in-vivo platforms, and
- studying nanowire function at the nanoscale in in-vitro platforms that integrate the minimum set of electroactive nanowire components.
This year, the researchers have developed in vivo platforms that demonstrated the role of the nanowires in the reduction of U(VI) as a mononuclear U(IV) mineral. They also identified outer membrane c-cytochromes foci surrounded by membrane regions of low uranium permeability. Genetic platforms were used to identify these components at the genetic level. Together with the nanowires, these outer membrane components reduce and immobilize uranium extracellularly and prevent its permeation and mineralization inside the cell envelope, thus preserving the cell’s vitality. In vivo studies also identified PpcA, a periplasmic c-cytochrome that is conserved in the Geobacteraceae family, as an important electron carrier across the cell envelope during nanowire-mediated uranium reduction. The researchers mass-produced PpcA in mature, redox-active form, immobilized it on self-assembled monolayers of alkanethiols on gold electrodes, and characterized it electrochemically. These studies demonstrate that this in vitro platform mimics the Geobacter’s periplasm. They also mass-produced and assembled recombinant nanowire subunits (pilins) in vitro to produce synthetic nanowires that mimic the native ones. These nanowires will be integrated with the periplasmic mimic to develop nanoplatforms and enable nanoscale studies of U(VI) reduction by native redox components in a controllable system. By integrating in vivo and in vitro platforms the research group is elucidating the mechanisms of uranium reduction by Geobacter bacteria, while providing the tools needed to optimize the in situ bioremediation of radionuclides and toxic metals via the stimulation of the native bacterial communities or the use of deployable, biocompatible nanodevices.