title: Nanoscale detection of organic signatures in carbonate microbialites.
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.
authors.affiliation: Institut de Minéralogie et de Physique des Milieux Condensés, Unité Mixte de Recherche 7590, Centre National de la Recherche Scientifique, University of Paris 6 and 7, 140 Rue de Lourmel, 75015 Paris, France. benzerar@impmc.jussieu.fr
authors.forename: Karim
authors.initials: K
authors.lastname: Benzerara
authors.forename: Nicolas
authors.initials: N
authors.lastname: Menguy
authors.forename: Purificación
authors.initials: P
authors.lastname: López-García
authors.forename: Tae-Hyun
authors.initials: TH
authors.lastname: Yoon
authors.forename: Józef
authors.initials: J
authors.lastname: Kazmierczak
authors.forename: Tolek
authors.initials: T
authors.lastname: Tyliszczak
authors.forename: François
authors.initials: F
authors.lastname: Guyot
authors.forename: Gordon E
authors.initials: GE
authors.lastname: Brown