Nanoscale Visualization of Biomineral Formation in Coral Proto-Polyps

Calcium carbonate platforms produced by reef-building stony corals over geologic time are pervasive features around the world [1]; however, the mechanism by which these organisms produce the mineral is poorly understood (see review by [2]). It is generally assumed that stony corals precipitate calcium carbonate extracellularly as aragonite in a calcifying medium between the calicoblastic ectoderm and pre-existing skeleton, separated from the overlying seawater [2]. The calicoblastic ectoderm produces extracellular matrix (ECM) proteins, secreted to the calcifying medium [3–6], which appear to provide the nucleation, alteration, elongation, and inhibition mechanisms of the biomineral [7] and remain occluded and preserved in the skeleton [8–10]. Here we show in cell cultures of the stony coral Stylophora pistillata that calcium is concentrated in intracellular pockets that are subsequently exported from the cell where a nucleation process leads to the formation of extracellular aragonite crystals. Analysis of the growing crystals by lattice light-sheet microscopy suggests that the crystals elongate from the cells’ surfaces outward. Using coral cell cultures, Mass et al. show that the stony coral biomineralization mechanism begins with intracellularly concentrated calcium, which is exported for extracellular crystal nucleation and growth. Aragonite crystals elongate outward from an extracellular protein matrix into the culture medium.