Biochemical characteristics of the soluble organic matrix from the shell of three recent terebratulid brachiopod species.
Résumé
To build their shells, brachiopods secrete a mixture of proteins and polysaccharides, collectively called the organic matrix. This matrix mediates the calcification process by allowing crystal nucleation followed by elongation and finally by stopping the crystal growth. Ultimately, the matrix controls the different microstructures formed. Brachiopod shells are composites with both organic and inorganic constituents intimately associated. This work represents an attempt to characterize the matrices using two combined approaches: (1) scanning electron microscopy for localization within the shell, (2) analysing extracted matrices at the molecular level to determine their biochemical properties. Representatives of the three terebratulid brachiopod genera: Gryphus , Liothyrella and Neothyris , were observed according to a standard procedure highlighting the microstructural details. Additionally, the acetic acid-soluble shell matrices were extracted and analyzed by sodium dodecyl polyacrylamide gel electrophoresis. Gels were subsequently stained with silver, Alcian blue and carbocyanine. In addition, the shell matrices were tested for their ability to bind calcium. Finally, functional in vitro assays were performed to check for the first time the effect of some of the shell matrices on calcium carbonate precipitation. Results show that the three matrices are extremely minor components ( < 0.05 wt %) of the shell and that the three genera exhibit some similarities in the electrophoretic patterns of their soluble matrices. Furthermore, the tested matrices drastically modify the morphology of calcite crystals in vitro. Putative similarities between brachiopod and molluscan shell matrices are discussed, as well as the early diagenesis effect that may affect brachiopod matrices.