The Skeletal Organic Matrix from Mediterranean Coral Balanophyllia europaea Influences Calcium Carbonate Precipitation

Abstract

Scleractinian coral skeletons are made mainly of calcium carbonate in the form of aragonite. The mineral deposition occurs in a biological confined environment, but it is still a theme of discussion to what extent the calcification occurs under biological or environmental control. Hence, the shape, size and organization of skeletal crystals from the cellular level through the colony architecture, were attributed to factors as diverse as mineral supersaturation levels and organic mediation of crystal growth. The skeleton contains an intra-skeletal organic matrix (OM) of which only the water soluble component was chemically and physically characterized. In this work that OM from the skeleton of the Balanophyllia europaea, a solitary scleractinian coral endemic to the Mediterranean Sea, is studied in vitro with the aim of understanding its role in the mineralization of calcium carbonate. Mineralization of calcium carbonate was conducted by overgrowth experiments on coral skeleton and in calcium chloride solutions containing different ratios of water soluble and/or insoluble OM and of magnesium ions. The precipitates were characterized by diffractometric, spectroscopic and microscopic techniques. The results showed that both soluble and insoluble OM components influence calcium carbonate precipitation and that the effect is enhanced by their co-presence. The role of magnesium ions is also affected by the presence of the OM components. Thus, in vitro, OM influences calcium carbonate crystal morphology, aggregation and polymorphism as a function of its composition and of the content of magnesium ions in the precipitation media. This research, although does not resolve the controversy between environmental or biological control on the deposition of calcium carbonate in corals, sheds a light on the role of OM, which appears mediated by the presence of magnesium ions.

Proteomic analysis of skeletal organic matrix from the stony coral Stylophora pistillata

“…Whereas the oral endodermal cells host endosymbiotic photosynthetic algae of the genus Symbiodinium, the aboral ectodermal cells, or calicoblastic cells, are the sites of biomineralization. Calicoblastic cells are thought to secrete SOM, which adheres the cells to recently formed extracellular skeleton, and are also considered to be intimately involved in nucleation and growth of aragonite crystals (23,24). SOM is retained in the coral skeleton and the amino acid composition of the protein fraction indicates that the SOM is distinct from cellular or mucus protein (25,26).…”

Polymorphs, Proteins, and Nucleation Theory: A Critical Analysis

“…In nature, invertebrate organisms primarily utilize calcium carbonates as the building materials for extracellular skeletal elements such as the mollusk shell [1][2][3][4] sea urchin spicules [5][6][7][8][9][10][11] and corals [12][13][14]. Biological calcium carbonates can exist in an amorphous state (amorphous calcium carbonate, ACC) [9][10][11][15][16][17][18][19] and as three different crystalline polymorphs: calcite, aragonite, and vaterite, where calcite is the more stable form and aragonite and vaterite are metastable relative to calcite [15,16].…”

“…The attraction of biological calcium carbonate polymorphism to the scientific community is that this process occurs largely under ambient conditions [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] and thus represents a "game changer" for materials science and chemistry communities who have utilized non-ambient and sometimes extreme conditions to generate a given polymorph. What is often overlooked in the discussion of biological calcium carbonate polymorphism is the role and identity of extrinsic agents in the formation and stabilization processes.…”

“…We must, however, also acknowledge that invertebrate skeletal elements are composite structures consisting of the mineral phase(s) and macromolecular components [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Here, the mineral phase nucleates and matures within this matrix.…”

“…Therefore the potential role of matrix macromolecules such as proteins looms large in polymorphism. If proteins do play a role in calcium carbonate polymorph selection and stabilization, then this process must fall under genomic and proteomic guidance [5][6][7][8]13,14,[26][27][28]. Hence, polymorphism could be controlled by the temporal, spatial, and quantitative aspects of protein components within the extracellular space.…”