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| Titel |
Scleractinian corals cultured in low Mg/Ca seawater form aragonite skeleton |
| VerfasserIn |
Jaroslaw Stolarski, Stéphanie Reynaud, Christine Ferrier-Pages, Katarzyna Janiszewska, Isabelle Domart-Coulon, Eric Beraud, Yves Marrocchi, Maciej Mazur, Jakub Szlachetko, Anders Meibom |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250040135
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| Zusammenfassung |
| Scleractinian corals represent a testing ground for ideas regarding biologically vs.
environmentally controlled calcification. The morphology of skeletal micro-structural units
(arrangement of the skeletal fibers) and their biogeochemical composition have, for a long
time, been interpreted from two opposite view points: (1) as a purely physico-chemical
process involving simple supersaturation of a fluid close in composition to seawater,
hypothesized to exist at the interface between the skeleton and the calicoblastic cell-layer, or
(2) a complete physiological control of calcification by the organism by means of a presumed
amorphous precursor phase and precisely utilized organic macromolecules that control
mineralogy, crystal orientation etc.
Paleontological data originally supported the second interpretation because the aragonitic
skeletal mineralogy appeared to be stable through geological time despite of changes in
seawater chemistry (e.g., the late Mesozoic decrease of Mg/Ca ratio), which was believed to
promote inorganic precipitation of calcite. However, Ries et al. (Geology 2006, 34: 525-528)
argued that scleractinians are so-called ‘hyper-calcifiers’ and limited in their mineralogical
control. Accordingly, in modern seawater (Mg/Ca molar ratio = 5.2) such organisms
form aragonite simply because the Mg/Ca ratio favors this mineralogy. However,
if the Mg/Ca ratio drops below 3.5, the mineralogy of such ‘hyper-calcifiers’ are
supposed to become calcitic. In low-Mg/Ca experiments, Ries et al. detected calcite by
X-ray diffraction of the bulk skeleton of Acropora, Montipora, and Porites and also
indicated, by electron microprobe analyses, the presence of calcite in the uppermost
portion of coral skeleton, though the exact position of the mapped areas were not
indicated.
We have cultured Acropora, Porites, Pavona and Galaxea in low Mg/Ca (compared with
normal seawater) artificial seawater (ASW). A low Mg/Ca ratio can be obtained either by
lowering the Mg concentration or by increasing the Ca concentration. In our experiments, we
did both. Before the corals were exposed to low Mg/Ca ASW conditions, they were labeled
with 86Sr for 3 days, following procedures described in Houlbreque et al. (GRL 2009:
doi:10.1029/2008GL036782), in order to precisely define the skeletal growth front at the start
of the experiment. 86Sr labeled corals were transferred to aquaria with ASW with Mg/Ca = 2.
In the experiment in which a low Mg/Ca ratio was obtained by lowering the Mg
concentration, the corals immediately died, presumably because Mg is an element required
in metabolism. In the experiment in which the low Mg/Ca ratio was obtained by
increasing the Ca concentration, the corals appeared unstressed (tentacles extended) and
they were kept alive for 30 days. Subsequent NanoSIMS imaging showed that the
86Sr-labelled growth front in Porites, Pavona, and Galaxea had been overgrown by a
ca. 10-20 μm newly formed skeleton. The 86Sr label was not incorporated into
Acropora specimens, most likely because skeleton formation had ceased during
handling, perhaps because of stress or sickness in this species. Micro-raman mapping
demonstrated an exclusively aragonitic mineralogy of the skeleton formed in the low
Mg/Ca ASW. This indicates that scleractinian corals form their skeleton under a
biologically control that renders the skeletal mineralogy insensitive to even large
variations in the Mg/Ca ratio of the ambient seawater. Additional experimental
observations, not mentioned here due to length limitations, will be presented at the meeting. |
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