Biochemical characteristics of the shell soluble organic matrix of some recent Rhynchonelliformea (Brachiopoda).
Résumé
To build up their shell, brachiopods secrete a mixture of proteins and polysaccharides, collectively called the shell organic matrix. This matrix is supposed to mediate the calcification process by allowing the crystal nucleation and elongation and finally, by stopping the crystal growth. The aim of this work is, firstly, to observe at nanoscale the close relationships between the mineral and the organic phases, and secondly, to characterize the acetic acid-soluble matrices extracted from different articulate brachiopods. Recent articulate brachiopod shells were observed according to a standard procedure, which emphasizes the microstructural details. For the biochemical characterization, the acetic acid-soluble shell matrices were extracted and analyzed by SDS-PAGE. The gels were subsequently stained with CBB, silver nitrate, Alcian Blue, PAS and carbocyanine. Furthermore, the matrices were tested for their ability to interact with the precipitation of calcite in vitro. On gels, the tested matrices exhibited some similarities, but also drastic differences. All the tested matrices modified, at different degrees, the morphology of calcite crystals. The true mineralogical nature of the early-secreted elementary carbonate calcium structures (granules) is discussed, as well as the properties of the soluble matrices compared to those of mollusks.