[cdn-nucl-l] Uranium 'pearls' before slime

["Adam McLean" <adam.mclean@utoronto.ca>]

Posted on the PNNL news list on August 7, 2006 and at:
www.pnl.gov/news/release?id=175

Adam

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From: pnnl.news@pnl.gov [mailto:pnnl.news@pnl.gov] 
Sent: Monday, August 07, 2006 8:00 PM
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Subject: PNNL News - Uranium 'pearls' before slime

Release date: August 7, 2006
Contact: Bill Cannon 
Uranium 'pearls' before slime
PNNL-led team discovers that bacteria roll out carpet of goo that converts
deadly heavy metal into less threatening nano-spheres

PEARL JAM - Soluble uranium has been converted to solid uraninite (UO2),
strung like tiny pearls along branching 'extracellular polymeric substance,'
or slime secreted by Shewanella bacteria. 

RICHLAND, Wash. - Since the discovery a little more than a decade ago of
bacteria that chemically modify and neutralize toxic metals without apparent
harm to themselves, scientists have wondered how on earth these microbes do
it.

For Shewanella oneidensis, a microbe that modifies uranium chemistry, the
pieces are coming together, and they resemble pearls that measure precisely
5 nanometers across enmeshed in a carpet of slime secreted by the bacteria.
The pearl is uranium dioxide, or uraninite, which moves much less freely in
soil than its soluble counterpart, a groundwater-contamination threat at
nuclear waste sites.

The U.S. Department of Energy estimates that uranium contaminates more than
2,500 billion liters of groundwater nationwide; over the past decade, the
agency has support research into the ability of naturally-occurring microbes
that can halt the uranium's underground migration to prevent it from
reaching streams used by plants, animals and people.

Assembling a battery of evidence, scientists have for the first time placed
the bacterial enzymes responsible for converting uranium to uraninite at the
scene of the slime, or "extracellular polymeric substance" (EPS), according
to a study led by the DOE's Pacific Northwest National Laboratory in today's
advance online edition of PLoS Biology.

"Shewanella really puts a lot of stuff outside the cell," said PNNL chief
scientist Jim Fredrickson, the study's senior author. "It's very tactile
compared with pathogens, which go into hiding to evade detection by the
immune system."

Another oddity is Shewanella's ability to "breathe," or reduce, metals the
way we human beings do oxygen. When oxygen is unavailable, Shewanella can
pass excess energy during respiration in the form of electrons to metal and
alter the metal's chemistry in the bargain-for instance, turning soluble
uranium into solid, insoluble uraninite (uranium dioxide).

Fredrickson, PNNL staff scientist/lead author Matthew Marshall and
colleagues wondered whether uranium-reducing components in that stuff
outside the cell, the EPS, might help Shewanella seek out and lock up heavy
metals.
To pose that question, which remains open, they first had to prove that the
same metal-reducing enzymes-proteins called c-type cytochromes-associated
with uraninite formation in the outer membrane could also be found outside
the cell in the EPS.

This they did through a variety of experiments that included creating mutant
strains unable to make outer-membrane cytochrome, or OMC, leading to an
excess of uraninite particles forming only inside the cell, in the periplasm
- the region between the microbe's cell and outer membrane. In nonmutants,
on the other hand, OMC and uraninite were found mainly outside the cell in
association with the EPS.

Collaborators from Argonne National Laboratory applied X-ray fluorescence
microscopy at the Advanced Photon Source to show that iron, which is also
found in OMC, was in the uraninite-EPS complex. Combining high-resolution
microscopy and OMC-specific antibodies, the researchers repeatedly found the
metal-reducing proteins in the uraninite-EPS complexes.

The authors noted that the OMC-containing EPS may be involved in the
transfer of electrons outside the cell or is possibly a way the microbes
shed the uraninite particles.

"Regardless," Fredrickson said, "the sticky EPS may behave like glue and
bind the uranium particles to soil, further impeding its migration in the
environment."

The research was funded by DOE's Office of Biological and Environmental
Research, Environmental Remediation Sciences Program and Genomics: Genomes
to Life. Part of this research was performed as a biogeochemistry grand
challenge at the W.R. Wiley Environmental Molecular Sciences Laboratory, a
DOE national user facility located at PNNL. 

PNNL is a DOE Office of Science laboratory that solves complex problems in
energy, national security, the environment and life sciences by advancing
the understanding of physics, chemistry, biology and computation. PNNL
employs 4,200 staff, has an annual budget of more than $725 million, and has
been managed by Ohio-based Battelle since the lab's inception in 1965.
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