Aretz et al_2011.pdf - ORBi - Université de Liège

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Kölner Forum Geol. Paläont., 19 (2011)
M. ARETZ, S. DELCULÉE, J. DENAYER & E. POTY (Eds.)
Abstracts, 11th Symposium on Fossil Cnidaria and Sponges, Liège, August 19-29, 2011
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Biological spherulite: Record of physiological activity in stony coral
skeleton
Jarosław STOLARSKI
Institute of Paleobiology, Polish Academy of Sciences, Warsaw, Poland; stolacy@twarda.pan.pl
The spherulitic model of the skeleton growth (BRYAN & HILL 1943) was a breakthrough in providing a
coherent mechanism of growth of the scleractinian skeleton and still remains an inspiration for some
geochemical models of skeleton formation (e.g., HOLCOMB et al. 2009). According to this model, formation
of the skeleton is explained in physico-chemical categories, the same that apply to the abiotic growth of
various spheroidal bodies consisting of radiating crystals. However, structural and geochemical similarity
between spherulites precipitated abiotically and those formed by organisms (including corals) is not
sufficient to invoke similar controlling mechanisms. Fish otoliths (spherulite-like calcium carbonate
structures involved in balance and hearing) provide an extreme example for the failure of such reasoning:
their radial-concentric structure (or its lack) and selection of calcium carbonate polymorph (aragonitic vs.
calcitic) is controlled entirely by expression levels of the starmaker gene (SÖLLNER et al. 2003). Although, the
role of distinct genes in the process of scleractinian coral skeletogenesis has not yet been precisely
elucidated (preliminary hints e.g., REYES-BERMUDEZ et al. 2009) there is an increasing number of evidences
suggesting the prevailing control of the structure and composition of the skeleton by the organism.
Scleractinian skeletons contain inclusions of organic macromolecules whose composition (acidic
glycoproteins) and distribution pattern suggest their formative role during the skeletogenesis rather than
passive entrapment in crystals which cyclic precipitation is driven by calcium carbonate saturation state.
Similarly, concentrations and micro-scale patterns of some trace elements (e.g., Mg) differ essentially from
those in minerals formed in equilibrium with the sea-water geochemistry (e.g., MEIBOM et al. 2008).
Histological observations of intact tissue-skeleton interface (e.g., TAMBUTTÉ et al. 2007) call into question the
presence of an Extracellular Calcifying Fluid (ECF) zone as postulated by some geochemical models. In
addition to the diversity in distribution patterns of Rapid Accretion Deposits („centers of calcification”)
which traditionally was considered as an evidence of limited organismal control on mineralization, there is
also large diversity in Thickening Deposits („fibers”) organization. Their distinct patterns are closely
associated with members of molecularly distinguished clades (e.g., KITAHARA et al. 2010; JANISZEWSKA et al.
2011). Such precise biological control of formation of the thickening deposits and lack of ECF space is also
suggested by skeleton isotope labeling experiments (HOULBREQUE et al. 2009).
Many of the above mentioned skeletal features that seems strongly biologically-influenced are evident
also in coralla of the azooxanthellate genus Desmophyllum which otherwise can be considered as perfect
model of “spherulitic” organization (Fig. 1). The goal of this presentation is to provide an overview of the
most comprehensive up to date structural and biogeochemical analysis of the skeleton of Desmophyllum and
dialectically confront competing arguments regarding the role of “biological” vs. “geological” factors in the
growth of the skeleton of this coral.
BRYAN, W.H & HILL, D. (1942): Spherulitic crystallization as a mechanism of skeletal growth in the Hexacorals. -
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HOLCOMB, M., COHEN, A.L., GABITOV, R.I. & HUTTER, J.L. (2009): Compositional and morphological features of aragonite
precipitated experimentally from seawater and biogenically by corals. - Geochimica et Cosmochimica Acta, 73:
4166-4179.
HOULBREQUE, F., MEIBOM, A., CUIF, J.P., STOLARSKI, J., MARROCCHI, Y., FERRIER-PAGÉS, C., DOMART-COULON, I. & DUNBAR,
R.B. (2009): Strontium-86 labeling experiments show spatially heterogeneous skeletal formation in the scleractinian
coral Porites porites. - Geophysical Research Letters, 36: L04604, doi:10.1029/2008GL036782.
JANISZEWSKA, K., STOLARSKI, J., BENZERARA, K., MEIBOM, A., MAZUR, M., KITAHARA, M. & CAIRNS, S.D. (2011): A unique
skeletal microstructure of the deep-sea micrabaciid scleractinian corals. - Journal of Morphology, 272: 191-203,
doi:10.1002/jmor.10906.
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