Growth dynamics and amorphous-to-crystalline phase transformation in natural nacre

Biominerals, such as nacreous bivalve shells, are important archives of environmental information. The processes explaining how the organisms code this information into the structure and composition of their shells are yet unknown. Most marine calcifiers form their shells from amorphous calcium carbonate by particle attachment and stepwise crystallisation of metastable precursor phases. However, the mechanism of this transformation including its incorporation of trace metal ions used for environmental reconstructions are poorly constrained. Using the nacreous shell of the Mediterranean mussel, we explore the formation of nacre from the meso- to the atomic scale. We use a novel combination of strontium pulse-chase labelling in aquaculture experiments and correlative micro- to sub-nanoscale analysis to show that nacre grows in a two-step process and crystallizes via localised dissolution and reprecipitation within nanogranules. Our findings elucidate how stepwise crystallization pathways affect trace element incorporation in natural biominerals, while preserving their intricate hierarchical structure.