EFFECTS OF AMINO ACIDS, MAGNESIUM, AND MOLLUSCAN EXTRAPALLIAL FLUID ON CRYSTALLIZATION OF CALCIUM CARBONATE: IN VITRO EXPERIMENTS

¹ Department of Zoology, Duke University, Durham, North Carolina 27706

The extrapallial fluid (EPF), the fluid component of the shell-forming system of molluscs, has been examined for its effect on the rate of CaCO₃ crystal formation in vitro. The medium was an artificial inorganic extrapallial fluid supersaturated with respect to CaCO₃. EPF of the bivalves Crassostrea virginica and Mercenaria mercenaria strongly inhibited CaCO₃ crystallization in high dilution. The inhibitory material had negatively charged groups as indicated by the removal of inhibition subsequent to passage through a DEAE-Sephadex column and the restoration of inhibition in material eluted from the column with 2.0 M NaCl. Polyaspartic and polyglutamic acids with COO^- groups also inhibited crystal formation whereas polyamino acids lacking those groups and free amino acids were without effect. The inhibition by the acidic polyamino acids suggests that polypeptides known to be present in EPF are one possible cause of inhibition of crystal formation by EPF. Mg present in EPF was also strongly inhibitory. The observed inhibition of in vitro crystallization by EPF and Mg indicates that these substances could be factors in governing CaCO₃ deposition rate in molluscan shell.

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