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| Titel |
Nanostructure and composition of bivalve shells |
| VerfasserIn |
D. E. Jacob, A. L. Soldati, R. Wirth, J. Huth, U. Wehrmeister, W. Hofmeister |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250025878
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| Zusammenfassung |
| Shells and pearls of unionid mussels (Hyriopsis cumingii, Margaritifera margaritifera,
Diplodon chilensis patagonicus) were studied by high resolution microbeam methods
and μ-computer tomography to gather insight into the nanostructure and chemical
composition of nacre and prism layers. Natural and cultured pearls are formed by many
mollusc species and their generation is very similar to that of shells resulting in
identical prismatic and nacreous structures of shells and pearls. Basic difference is,
however that pearl culturing methods induce biomineralisation of CaCO3 around a
crystalline bead which results in a reverse structural organisation compared to bivalve
shells.
Bivalve shell growth starts from a thick organic matrix (the periostracum; Eyster and
Morse, 1984) which is followed towards the inside by two variously thick layers consisting of
prismatic CaCO3 aggregations and layers of CaCO3 platelets, respectively. Platelets
and prisms are individually covered by a chitinous organic matrix which lends
structural support and is thought to exert control over the mineralization process. The
minerals within the organic sheaths are highly-aligned poly-twinned crystals with
a slightly distorted lattice due to inclusions of organic molecules (Pokroy et al.,
2006).
Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM),
Atomic Force Microscopy (AFM) and Raman Microscopy analyses of the shells and
pearls show that both structures, prisms and platelets, consist of nanometre-sized
organic membrane-coated granules of CaCO3 (Jacob et al., 2008). In the vicinity of
the periostracum, the granules consist of amorphous calcium carbonate (ACC),
but the crystallinity increases with increasing distance from the periostracum. The
transition from disordered (amorphous) to crystalline CaCO3 is gradual within a few
micrometers and coincides with a decrease in porosity. Concentrations of sulphur and
phosphorus are higher in ACC than in aragonite indicating a higher organic content of
ACC.
Bivalve larval shells were shown to consist entirely of ACC before this phase crystallizes
to form aragonite (Weiss, et al., 2002). The occurrence of ACC in pearls and adult shells
close to the periostracum reported here and by Jacob et al. (2008) is taken as evidence that
bivalve shell formation starts from ACC secreted in organic vesicles. Lately, a number of
studies reported similar granular nanostructures for many different mollusc species which
implies that shell growth by secretion of ACC vesicles could be a widespread phenomenon in
biology.
Vaterite could be identified in freshwater cultured pearls as well as in shells of
Hyriopsis cumingii and Diplodon chilensis patagonicus. Aragonite and vaterite
were found to coexist and are crosscut by growth lines, implying simultaneous
formation. In pearls, it was found that vaterite, like aragonite, forms from ACC
(Jacob et al., 2008) and is therefore not the precursor phase of aragonite in this
system.
Eyster L.S. and Morse M. P. (1984). Early shell formation during molluscan
embryogenesis, with new studies on the Surf clam, Spisula solidissima. Am. Zoologist 24:
871-882.
Jacob, D.E., A.L. Soldati, R. Wirth, J. Huth, U. Wehrmeister und W. Hofmeister (2008).
Nanostructure, chemical composition and mechanisms of bivalve shell growth. Geochim.
Cosmochim. Acta, 72, 22, 5401-5415.
Pokroy B., Fitch A. N., Lee P. L., Quintana J. P., Caspi E. N., and Zolotoyabko E. (2006)
Anisotropic lattice distortions in the mollusk-made aragonite: A widespread phenomenon. J.
Structural Biology 153, 145-150.
Weiss I. M., Tuross N., Addadi L., and Weiner S. (2002) Mollusc larval shell formation:
Amorphous calcium carbonate is a precursor phase for aragonite. J. Exp. Zoology 293,
478-491. |
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