Complex morphologies of biogenic crystals emerge from anisotropic growth of symmetry-related facets

Directing crystal growth into complex morphologies is challenging, as crystals tend to adopt thermodynamically stable morphologies. Yet, many organisms form crystals with intricate morphologies, as exemplified by coccoliths, microscopic calcite crystal-arrays produced by unicellular algae. The complex morphologies of the coccolith crystals were hypothesized to materialize from numerous crystallographic facets, stabilized by fine-tuned interactions between organic molecules and the growing crystals. Using state-of-the-art electron tomography, we examined multiple stages of coccolith development in 3D. We found that the crystals are expressing only one set of symmetry-related crystallographic facets, which grow differentially to yield highly anisotropic shapes. Morphological chirality arises from positioning the crystals along specific edges of these same facets. Our findings show that manipulations of growth kinetics can yield complex crystalline morphologies.