INORGANIC OVERGROWTH OF ARAGONITE ON MOLLUSCAN NACRE EXAMINED BY ATOMIC-FORCE MICROSCOPY

The nacre (mother-of-pearl) that forms the irridescent inner layers of mollusc shells is a highly ordered microlaminate composite of aragoni te crystals and biopolymers with a strength and fracture resistance th at far exceed those of the mineral crystals themselves. The processes governing the biofabrication of this material by the secretory cells o f the mantle are complex and only partially understood. We have used t he atomic force microscope (AFM) to investigate the aqueous solution c onditions under which mineral growth can occur on the nacreous layer o f the shell of the bivalve mollusc Atrina sp. In situ imaging of the m ature nacre surface exposed to a pH-controlled environment of natural seawater with added carbonate ions reveals that inorganic overgrowth o f aragonite can occur within the ranges of pH and inorganic ion concen trations found in the molluscan extrapallial fluid from which the mine ral is produced during biological shell growth. Thus, we posit that on ce nucleation has occurred, nacreous tablets could grow inorganically in the extrapallial space; the role of proteins and other macromolecul es may be limited to initiating growth or controlling morphology throu gh selective adsorption and spatial constraint on the growing crystal.