Templateless crystallization of holococcolith crystals visualized by intracellular site-specific three-dimensional microscopy.

Biological crystallization generates some of the most intricate and diverse materials found in nature. Among the most striking examples are calcium carbonate structures called coccoliths, which are formed by unicellular marine algae. Despite the exquisite control over crystal orientations and arrangements, the mechanisms by which these cells control crystal nucleation remain poorly understood, largely due to the experimental difficulty of probing crystallization in vivo. Here, we report on intracellular crystallization during the formation of holococcoliths-superstructures assembled from rhombohedral calcite crystals. We establish a serial cryo-focused ion beam milling strategy to directly access the intracellular site where calcite precipitates. Cryo-electron tomography (cryoET) demonstrates a biomineralization mode where nucleation and growth of crystals occur within an isotropic environment and in the absence of any templating structures discernable by cryoET. Based on these observations, we propose a two-step mechanism, where an ordering step follows the initial random nucleation events. Such a process might be common to other biomineralization phenomena that evolve their order gradually.