- Number 322 |
- October 11, 2010
Scientists investigate long-distance bioreduction
Shewanella oneidensis produced
rosettes of iron(II)phosphate in
the simulated soil. The bacteria
produced these structures
without directly contacting
the iron oxides.
The ground beneath our feet is alive with countless microorganisms that go about their lives engaged in transferring electrons to iron oxides. Called “bioreduction,” this microscopic activity changes the oxidation state of iron oxides, affecting its ability to trap contaminants from surrounding soil, sediment and subsurface materials.
To gain a better understanding of the mechanics of bioreduction and the impacts that this process has on subsurface environmental remediation, scientists at DOE’s Pacific Northwest National Laboratory recently developed a new system to study the phenomenon of microbial transfer of electrons without direct contact with iron oxides.
In nature, bioreduction occurs either by direct microbe-to-iron oxide contact or by indirect mechanisms involving soluble biogenic reactants. The iron oxides are commonly embedded within microscopic intragranular pores and microfractures, just out of reach of eager microbes. PNNL scientists simulated these subsurface conditions by embedding nanoparticles of iron oxide into grains of highly porous silica. In a series of controlled experiments, researchers encouraged bioreduction with Shewanella oneidensis MR-1 in numerous anoxic conditions and with the introduction of various chemicals, such as phosphate.
The resulting experiments demonstrated that microbes do transfer electrons without direct contact with iron oxide. While researchers remain speculative about how the microbes accomplish this transfer, they were surprised by the byproduct of the bioreduction process – blooms of iron(II) phosphate precipitates as large as 20 to 30 microns growing on the surface and within the silica grains. “We were not expecting to find these structures,” said PNNL scientist Tanya Peretyazhko, adding that the iron(II) phosphates were densely colonized by microbes. “This discovery was fascinating and surprising.”
Peretyazhko and the PNNL research team developed techniques to preserve the bioreduced samples for X-ray diffraction and scanning and transmission electron microscopy. This analysis answered questions about mineralogy and morphology of iron precipitates, as well as the distribution of microorganisms on the surface of porous silica. With the simulation technique established, EMSL scientists will next conduct research to fully understand the mechanism that microbes use to reduce iron oxides without direct contact, as well as mechanisms of precipitate formation for this complex system.
According to Peretyazhko, this research adds to a growing base of knowledge on bioreduction and biomineralization mechanisms. In the future it may prove important for studies of water quality, fate of contaminants and the development of more effective subsurface environmental cleanup strategies.
This work was supported by the Department of Energy’s Office of Basic Energy Sciences and the Department of Energy’s Office of Biological and Environmental Research. — Kristin Manke
Submitted by DOE's Pacific Northwest National Laboratory