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 aragonite crystals and biopolymers with a strength and fracture resistance that far exceed those of the mineral crystals themselves. The processes governing the biofabrication of this material by the secretory cells of the mantle are complex and only partially understood. We have used the atomic force microscope (AFM) to investigate the aqueous solution conditions under which mineral growth can occur on the nacreous layer of the shell of the bivalve mollusc Atrina sp. In situ imaging of the mature nacre surface exposed to a pH-controlled environment of natural seawater with added carbonate ions reveals that inorganic overgrowth of aragonite can occur within the ranges of pH and inorganic ion concentrations found in the molluscan extrapallial fluid from which the mineral is produced during biological shell growth. Thus, we posit that once nucleation has occurred, nacreous tablets could grow inorganically in the extrapallial space; the role of proteins and other macromolecules may be limited to initiating growth or controlling morphology through selective adsorption and spatial constraint on the growing crystal.