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| Titel |
Cyanobacterial calcification in modern microbialites at the submicrometer scale |
| VerfasserIn |
E. Couradeau, K. Benzerara, E. Gerard, I. Estève, D. Moreira, R. Tavera, P. López-García |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 10, no. 8 ; Nr. 10, no. 8 (2013-08-01), S.5255-5266 |
| Datensatznummer |
250018373
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| Publikation (Nr.) |
 copernicus.org/bg-10-5255-2013.pdf |
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| Zusammenfassung |
| The search for microfossils in the geological record has been a long-term
challenge. Part of the problem comes from the difficulty of identifying such
microfossils unambiguously, since they can be morphologically confused with
abiotic biomorphs. One route to improve our ability to correctly identify
microfossils involves studying fossilization processes affecting bacteria
in modern settings. We studied the initial stages of fossilization of
cyanobacterial cells in modern microbialites from Lake Alchichica (Mexico), a
Mg-rich hyperalkaline crater lake (pH 8.9) hosting currently growing
stromatolites composed of aragonite [CaCO3] and hydromagnesite
[Mg5(CO3)4(OH)2 · 4(H2O)]. Most of the biomass
associated with the microbialites is composed of cyanobacteria. Scanning
electron microscopy analyses coupled with confocal laser scanning microscopy
observations were conducted to co-localize cyanobacterial cells and
associated minerals. These observations showed that cyanobacterial cells
affiliated with the order Pleurocapsales become specifically encrusted within
aragonite with an apparent preservation of cell morphology. Encrustation
gradients from non-encrusted to totally encrusted cells spanning distances of
a few hundred micrometers were observed. Cells exhibiting increased levels of
encrustation along this gradient were studied down to the nm scale using a
combination of focused ion beam (FIB) milling, transmission electron
microscopy (TEM) and scanning transmission x-ray microscopy (STXM) at the C,
O and N K-edges. Two different types of aragonite crystals were observed: one
type was composed of needle-shaped nano-crystals growing outward from the
cell body with a crystallographic orientation perpendicular to the cell wall,
and another type was composed of larger crystals that progressively filled
the cell interior. Exopolymeric substances (EPS), initially co-localized with
the cells, decreased in concentration and dispersed away from the cells while
crystal growth occurred. As encrustation developed, EPS progressively
disappeared, but remaining EPS showed the same spectroscopic signature. In
the most advanced stages of fossilization, only the textural organization of
the two types of aragonite recorded the initial cell morphology and spatial
distribution. |
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