Nanoscale detection of organic signatures in carbonatemicrobialites

Microbialites are sedimentary deposits associated withmicrobialmat communities and are thought to be evidence of some of theoldest life on Earth. Despite extensive studies of such deposits, littleis known about the role of microorganisms in their formation. Inaddition, unambiguous criteria proving their biogenicity have yet to beestablished. In this study, we characterize modern calcareousmicrobialites from the alkaline Lake Van, Turkey, at the nanometer scaleby combining x-ray and electron microscopies. We describe a simple way tolocate microorganisms entombed in calcium carbonate precipitates byprobing aromatic carbon functional groups and peptide bonds. Near-edgex-ray absorption fine structure spectra at the C and N K-edges provideunique signatures for microbes. Aragonite crystals, which range in sizefrom 30 to 100nm, comprise the largest part of the microbialites. Thesecrystals are surrounded by a 10-nm-thick amorphous calcium carbonatelayer containing organic molecules and areembedded in an organic matrix,likely consisting of polysaccharides, which helps explain the unusualsizes and shapes of these crystals. These results provide biosignaturesfor these deposits and suggest that microbialorganisms significantlyimpacted the mineralogy of Lake Vancarbonates.