LAUSR

Directing crystal growth into complex morphologies is challenging, as crystals tend to adopt thermodynamically stable morphologies. However, 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 electron tomography, we examined multiple stages of coccolith development in three dimensions. We found that the crystals express 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 suggest that growth rate manipulations are sufficient to yield complex crystalline morphologies. Description Asymmetrical crystal growth The shapes of crystalline materials reflect the growth rates of different faces, such as elongation in the fastest-growing direction. In the absence of impurities, boundary walls, or guiding macromolecules, one would expect symmetrical faces to grow at the same rate. Avrahami et al. examined crystal growth and arrangement in developing coccoliths, microscopic calcite crystal arrays produced by the unicellular alga Calcidiscus leptoporus (see the Perspective by Prywer). The authors found that the {104} faces connected by symmetry relations did not show the same growth rates, thus leading to symmetry breaking in growth and the formation of complex growth habits. This asymmetrical growth is not caused by guide macromolecules, but rather is solely controlled by the diffusion of ions. —MSL Organisms sculpt elaborate crystals within the confinement of the cell by using simple yet unconventional principles.