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
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
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.
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."
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
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.
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
"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
DOE/Pacific Northwest National Laboratory