Microplastics impact shell and pearl biomineralization of the pearl oyster Pinctada fucata.

Microplastics are extremely widespread aquatic pollutants that severely detriment marine life. In this study, the influence of microplastics on biomineralization was investigated. For the first time, multiple forms and types of microplastics were detected and isolated from the shells and pearls of Pinctada fucata. According to the present study, the abundance of microplastics in shells and pearls was estimated at 1.95 ± 1.43 items/g and 0.53 ± 0.37 items/g respectively. Interestingly, microplastics were less abundant in high-quality round pearls. Microplastics may hinder the growth of calcite and aragonite crystals, which are crucial components required for shell formation. During the process of biomineralization microplastics became embedded in shells, suggesting the existence of a novel pathway by which microplastics accumulate in bivalves. After a 96-h exposure to microplastics, the expression level of typical biomineralization-related genes increased, including amorphous calcium carbonate binding protein (ACCBP) gene which experienced a significant increase. ACCBP promotes the formation of amorphous calcium carbonate (ACC), which is the pivotal precursor of shell formation-related biominerals. ACCBP is highly expressed during the developmental stage of juvenile oysters and the shell-damage repair process. The increased expression of ACCBP suggests biomineralization is enhanced as a result of microplastics exposure. These results provide important evidence that microplastics exposure may impact the appearance of biominerals and the expression of biomineralization-related genes, posing a new potential threat to aquatic organisms.

A novel matrix protein PfX regulates shell ultrastructure by binding to specific calcium carbonate crystal faces.

Here, we have identified a novel matrix protein, named PfX, from the pearl oyster Pinctada fucada, and investigated the effects of recombinant PfX protein on calcium carbonate crystallization. The expression of PfX was spatially concentrated in the mantle tissue and gill, the former of which is responsible for the formation of shell structures. The shell notching assay showed a PfX expression response during injured shell repair and regeneration, suggesting the potential involvement of this matrix protein in shell biomineralization. Further, an in vitro crystallization assay showed that PfX could alter the CaCO3 morphologies of both calcite and aragonite polymorphs. Correspondingly, a binding assay indicated that PfX has strong binding affinity for CaCO3 crystals, especially aragonite. Further, the protein's calcite binding capacity increased obviously when particular crystal faces were induced. In addition, PfX conjugated with fluorescent dye cyanine-5 (cy5) was preferentially distributed on rough crystal faces instead of the smooth and common (1 0 4) faces of calcite during the crystallization. These results suggest that matrix protein PfX might regulate CaCO3 morphology via selective binding and inhibit the growth of certain crystal faces, providing new clues for understanding biomineralization mechanisms in mollusk.