**Applications of STXM to Microbial Bioweathering and
Biomineralization**

Karim Benzerara^1,2^, Tae Hyun Yoon^1,3^, Thomas Borsch^1^, Carmen
Cordova^4^, Nicolas Menguy^2^, Tolek Tyliszczak^5^, Alfred M.
Spormann^4^, and [Gordon E. Brown, Jr.]{.underline}^1,6^

^1^Surface & Aqueous Geochemistry Group, Department of Geological and
Environmental Sciences, Stanford University, Stanford, CA 94305-2115,
USA

^2^Laboratoire de Minéralogie-Cristallographie, UMR 7590 CNRS and
Institut de Physique du Globe de Paris, 4 place Jussieu, 75252 Paris
Cedex, France

^3^Department of Chemistry, Hanyang University, 17 Haengdang-dong,
Seongdong-gu,

Seoul, 133-791, Korea

^4^Department of Civil & Environmental Engineering, Stanford University,

Stanford, CA 94305, USA

^5^Chemical Sciences Division, Lawrence Berkeley National Laboratory,

Berkeley, CA 94720, USA

^6^Stanford Synchrotron Radiation Laboratory, SLAC, 2575 Sand Hill Road,

Menlo Park, CA 94025, USA

We have used the scanning transmission x-ray microscope on Molecular
Environmental Science branch line 11.0.2.2 at the ALS to study
bioweathering and biomineralization processes in several
microbial-mineral systems. These microbially mediated processes are
common and have major impacts on mineral weathering and formation in
many natural and man-made environments. In our first study, we examined
bioweathering of a meteoritic orthopyroxene
\[(Mg~0.75~Fe~0.23~Ca~0.01~)(Si~0.99~Al~0.01~)O~3~\] from a Martian
meteorite that fell in the southern Tunisian desert near Tatahouine in
1931. Filamentous bacteria of unknown taxonomy populated the surfaces of
the orthopyroxenes during a portion of the 70-year exposure period and
resulted in an amorphous Al-rich layer beneath the microorganism,
calcium carbonates of unique morphology associated with polysaccharides
adjacent to the microorganism, and regions surrounding the microorganism
with different iron oxidation states. These results indicate the
presence of unique microenvironments in the vicinity of the
microorganism that are not predicted by equilibrium thermodynamics. In a
second study, we characterized biominerals formed when *Caulobacter
crescentus* cells were incubated for three weeks in a calcification
medium. STXM spectromicroscopy at the C K-edge and P L-edge showed that
carbonate hydroxyapatite rather than a calcium carbonate polymorph
formed. In a third study, we examined C and N K-edge spectroscopic
biosignatures produced during the formation of a modern microbialite
deposit in Lake Van, Turkey, which is one of the largest alkali lakes in
the world. Nanometer-sized aragonite crystals were found embedded in an
organic matrix dominated by polysaccharides, which helps explain the
unusual morphology of the aragonite crystals. Our results question
existing models for the role of microorganisms in calcium carbonate
precipitation and provide new spectroscopic biosignature for these types
of deposits, which can be applied to ancient microbialites. In a fourth
study, we used STXM to examine the interaction of *Shewanella
putrifaciens* CN32 with Fe(III)-hydroxide nanoparticles. This
dissimilatory reducing bacterium uses Fe(III)-containing nanoparticles
as terminal electron acceptors which results in the reduction of Fe(III)
to Fe(II) and dissolution of the Fe(III)-hydroxide nanoparticles. Our
STXM results suggest that Fe(II)-carbonate biomineralization occurs and
shows direct evidence for Fe(III) reduction in distinct
microenvironments along the length of the *C. crescentus* cells.