The Role of Macromolecular Condensates in the Regulation of Intracellular Calcium Transport for Coccolith Formation

Inorganic minerals that form via regulated biological processes exhibit remarkable properties. This is due to the involvement of macromolecules that control biomineralization. Even though the interactions of these biopolymers with solid mineral phases are intensely studied, not much is known about their involvement in the preceding steps of intracellular transport of the mineral building blocks. In this work, the model system of coccolith calcite crystallization is utilized to address the role of mineral-associated polysaccharides in the transport of calcium ions. State-of-the-art cryo-electron tomography is used to image in situ ion-rich dense phases in the wild-type and in two mutant strains, defected in coccolith production. The results show that the abundance and solubility of the calcium-rich condensates need to be finely tuned for proper crystallization. When the native macromolecular assemblage is compromised, calcium is still present in the calcifying fluid as a solute, but this is not sufficient for coccolith development. These results suggest that biomineralizing systems achieve superior regulation of crystallization due to the use of dense macromolecule-rich phases.