scatterings

**Infrared Beamline Reveals Toxin-Reducing Microbes**

**L**awrence Berkeley Lab scientists have discovered a bac-teria that
thrives on toxic waste. A Fourier-transform infrared (FTIR)
spectromicroscopy beamline (Beamline 1.4.3) was implemented to identify
the toxin-devouring microbes. The bacteria detected converts hexavalent
chromium (Cr^6+^) a widespread industrial contaminant, into a safer,
stable compound (Cr^3+^). "Basically, the bugs eat the waste," said Paul
Preuss, a spokesperson for Lawrence Berkeley Lab.

Hoi-Ying Holman, along with colleagues Dale Perry, Michael Martin, Wayne
McKinney, and Jennie Hunter­Cevera, made the discovery by examining core
samples from 75 meters beneath a toxic waste containment site in Idaho.
By using infrared spectromicroscopy, they were able to follow the
reduction of toxic material among popula­tions of living organisms on
minerals: Lawrence Berkeley reports this is die first time that
biogeochemical transfor­mation of Cr^6+^ on a mineral surface has been
nondestruc­tively monitored and studied.

![](media/image1.png){width="2.1in" height="4.909722222222222in"}Since
infrared light does not kill bacteria, the transfor­mation of Cr^6+^
could be monitored as it occurred. Dis­tinct infrared absorption bands
served dual func­tions\--as chemical markers to detect different chromium
species, and as biological markers to detect the presence and activity
of microorgan­isms on specimen surfaces. The brightness of the infrared
radiation from the beam-line also made spatially resolved
spectromicro-scopy possible.

Previously, two mecha­nisms for reducing Cr^6+^ compounds were
postulat-ed. The biological mech-anism re­quires the presence of
mi­croorganisms to aerobically reduce the Cr^6+^. The chemical mechanism
relies on metal oxides, such as Fe(II) com­pounds, to catalyze the Cr^6+^
reduction reaction. Working with an infrared beamline over a five-day
period, the re­searchers applied Fourier­ transform spectromicroscopy to
observe the steps in the reduction process and the pre­cise location of
the reduced chromium. In addition, the effects on the biotic reduction
process of common organic co-contaminants, such as toluene vapor, were
evaluated.

"The infrared is the end of the spectrum not usually as­sociated with
synchrotrons," said Holman, "but for us it\'s perfect\--and not only
because it\'s non-destructive of or­ganisms. You have an extremely
complicated spectrum in the 10 μm region \[the dimension of the beam\].
We identi­fied markers in this spectral region that tracked the key
compounds that undergo changes. We could resolve the spectrum in time,
to follow the different steps of the reduc­tion, and also in space, to
see exactly where the reactions were happening."

On the magnetite samples with no living bacteria, no significant changes
were evident. On samples with living microorganisms *(Arthrobacter
oxydans)* in the absence of toluene, a weak chromium reduction was
detected. How-ever, when the *Arthrobacter oxydans* had been exposed to
toluene, FTIR spectromicroscopy revealed that hexavalent chromium and
toluene had been replaced by pentavalent chromium and hydrocarbon
degradation in association with biomolecules. This reduction of Cr^6+^
occurred direct­ly at the sites of bacterial concentration. Imaging the
sur­face at characteristic absorption bands demonstrated a strong
correlation between peak depletion of Cr^6+^ and toluene, and peak
concentration of biological molecules.

To determine if this microbial reduction process could occur in real
geologic samples, Holman exposed compos­ite mineral surfaces of basalt
rock chips containing resi­dent communities of microbes to solutions of
Cr^6+^ and toluene vapor. After four months, FTIR spectromicroscopy
showed that chromium-tolerant and chromi­um-reducing natural
microorganisms were thriving in association with Cr^3+^ (see figure).

"As far as we know," says Holman, "this is the first time that infrared
synchrotron studies have been used to follow the steps in the
transformation of toxic chromium on min­eral surfaces."

Although there are other microbes known to trans­form toxic heavy metals,
*Arthrobacter oxydans* is the first known type of bacteria able to live
deep underground. Holman\'s research has uncovered a way to monitor
toxic waste disposal which could represent an advance over current
methods. The FTIR method can now be expand­ed to examine other
infrared-amenable microbial/chemi­cal contaminant systems. Says Holman,
"This should help in the design and implementation of new,
environ­mentally benign remediation techniques for cleaning up
mixed-waste sites."