Chemistry of precious coral: Biomineraliztion

 
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Chemistry of precious coral: Biomineraliztion
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 The axis of precious corals is inorganic minerals synthesized by a process known as biomineralization . Precious corals take in calcium ions and bicarbonate ions, major constituents dissolved in seawater, and form calcite, one form of carbonate calcium (Table 1). A number of microscopic sclerites (Fig. 2) and tree-like axes are both made of calcite.
 The axis of precious corals is made into jewelry because of its beautiful luster when polished, coming in the various shades of red, pink and white. A red color component has been identified as a carotenoid called canthaxanthin (Fig. 3). Carotenoids are among the most abundant pigments in nature and found in vegetables such as carrots, tomatoes and pumpkins as well as in animals such as feathers of flamingos, salmons, trout and spiny lobsters. Meanwhile, the rigid axis of reef-building corals is aragonite, a different form of carbonate calcium (Table 1). Aragonite is too porous and rough to be used as gemstones.
 During biomineralization in precious corals, not the major element of carbonate calcium but trace elements, mainly alkaline-earth metals such as magnesium, strontium and barium as well as sulfate ions, are incorporated into the axis from surrounding waters as impurities. The amount of inorganic trace elements contained in the axis change regularly in the growth process that occurs from the center outwards. As a result, layers are formed in concentric circles resembling growth rings (Fig. 4). The composition ratio of inorganic trace elements in the axis also varies with habitats and seawater components. Looking at Mg/Ca and Ba/Ca ratios in the precious coral axis from major habitats, the composition ratios of them are different according to habitats and each has values characteristic of its locality (Fig. 5). Trace elements in precious corals are, therefore, expected to be used as important scientific data indicating their habitats and harvested areas.  (Hiroshi Hasegawa of Kanazawa University)

Glossary Biomineralization (Fig. 1): the process by which organisms form minerals. Aggregates of inorganic minerals are referred to as “hard tissues.” Familiar examples include corals, shells, pearls, bones, teeth, and diatom earth.

 [References]
●Hasegawa, H. and Yamada, M., 2010. Chemical analyses of carbonate skeletons in precious corals. In: Iwasaki, N., ed., Biohistory of Precious Coral: Corals Scientific Cultural and Historical Perspectives. Tokai University Press, Hadano, Japan, pp. 43?60.
●Hasegawa, H., Rahman. M. A., Luan. N.T., Maki. T. and Iwasaki. N., 2012. Trace elements in Corallium spp. as indicators for origin and habitat. Journal of Experimental Marine Biology and Ecology, 414-415, 1?5.
●Iwasaki, N. and Suzuki, T., 2010. Biology of precious coral. In: Iwasaki, N., ed., Biohistory of Precious Coral: Corals Scientific Cultural and Historical Perspectives. Tokai University Press, Hadano, Japan, pp. 3-25.

Fig.1. Biomineralization
Fig.1

Table 1. Crystal systems of carbonate calcium

  Calcite Aragonite
Crystal system traigonal rhombic
Density (g/cm3) 2.7 2.9
Mohs hardness 3.0 3.5 ? 4.0
スペース


Fig. 3
Fig. 3. Color components of precious corals: until mid-20th century, iron salts were considered as a red color component of precious coral axes. However, a carotenoid called canthaxanthin is now known to be responsible for the color. Precious corals are incapable of synthesizing geranylgeranyl-diphosphate, a substance essential for carotenoid formation. Therefore they obtain carotenoids from particulate organic matter including dead phytoplankton in their diet.
 


Fig. 2
Fig. 2. Electron micrograph of sclerites in the coenenchyme: The coenechyme of precious corals is imbedded with sclerites about 30-60 micrometers in diameter, made of calcite. The axis of precious corals is also made of calcite but the association of axial formation and sclerites is not known (Iwasaki and Suzuki, 2008).

Fig. 4
Fig. 4. Micrograph showing a cross section of a white coral (left) and its magnesium distribution (right): Growth rings can be analyzed accurately using the magnesium distribution. The sustainability of precious corals as natural resources is investigated using growth rates calculated from growth rings (Hasegawa et al. 2012).
  Fig. 5
Fig. 5. Relationship between Mg/Ca and Ba/Ca ratios in the precious coral axis: the composition of inorganic trace elements varies from one habitat to another. In other words, the area where a precious coral was harvested can be identified through the chemical analysis of inorganic elemental compositions (Hasegawa et al., 2012).