Nanoscale detection of organic signatures in carbonate microbialites

doi:10.1073/pnas.0603255103

Nanoscale detection of organic signatures in carbonate microbialites

^†Institut de Minéralogie et de Physique des Milieux Condensés, Unité Mixte de Recherche 7590, Centre National de la Recherche Scientifique, Department de Minéralogie, Institut de Physique du Globe de Paris, University of Paris 6 and 7, 140 Rue de Lourmel, 75015 Paris, France;
^‡Surface and Aqueous Geochemistry Group, Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115;
^¶Unité d’Ecologie, Systématique et Evolution, Unité Mixte de Recherche 8079, Centre National de la Recherche Scientifique, Université Paris-Sud, 91405 Orsay Cedex, France;
^‖Department of Chemistry, Hanyang University, Music 17 Haengdang-dong, Seongdong-gu, Seoul, 133-791, Korea;
^††Institute of Paleobiology, Polish Academy of Sciences, Twarda 51/55, 00818 Warsaw, Poland;
^‡‡Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720; and
^§§Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, MS 69, 2575 Sand Hill Road, Menlo Park, CA 94025

Communicated by W. G. Ernst, Stanford University, Stanford, CA, April 21, 2006 (received for review January 12, 2006)

Abstract

Microbialites are sedimentary deposits associated with microbial mat communities and are thought to be evidence of some of the oldest life on Earth. Despite extensive studies of such deposits, little is known about the role of microorganisms in their formation. In addition, unambiguous criteria proving their biogenicity have yet to be established. In this study, we characterize modern calcareous microbialites from the alkaline Lake Van, Turkey, at the nanometer scale by combining x-ray and electron microscopies. We describe a simple way to locate microorganisms entombed in calcium carbonate precipitates by probing aromatic carbon functional groups and peptide bonds. Near-edge x-ray absorption fine structure spectra at the C and N K-edges provide unique signatures for microbes. Aragonite crystals, which range in size from 30 to 100 nm, comprise the largest part of the microbialites. These crystals are surrounded by a 10-nm-thick amorphous calcium carbonate layer containing organic molecules and are embedded in an organic matrix, likely consisting of polysaccharides, which helps explain the unusual sizes and shapes of these crystals. These results provide biosignatures for these deposits and suggest that microbial organisms significantly impacted the mineralogy of Lake Van carbonates.

Footnotes

^§To whom correspondence should be addressed. E-mail: benzerar{at}impmc.jussieu.fr
Author contributions: K.B., P.L.-G., J.K., and G.E.B. designed research; K.B., N.M., P.L.-G., and T.-H.Y. performed research; T.T. contributed new reagents/analytic tools; K.B. and N.M. analyzed data; and K.B., P.L.-G., F.G., and G.E.B. wrote the paper.
Conflict of interest statement: No conflicts declared.
Abbreviations:

Abbreviations:

ACC,

amorphous calcium carbonate;

EELS,

electron energy loss spectroscopy;

EPS,

extracellular polymeric substances;

NEXAFS,

near-edge x-ray absorption fine structure;

STXM,

scanning transmission x-ray microscopy;

TEM,

transmission electron microscopy.