Spatial Relations between Coccoliths and Their Confining Membrane During Crystal Morphogenesis

Coccoliths are multicrystalline calcite structures formed by microalgae within an intracellular vesicle. The morphology of each crystal is complex, and recent studies postulate that coccolith morphogenesis is regulated by the bounding membrane of its vesicle. However, the limited information about the native-state organization within the cell makes it difficult to understand which structural aspects of the membrane are responsible for morphogenesis. Here, we examined the vesicular environment during the formation of Calcidiscus leptoporus coccoliths, using advanced cryo-electron microscopy and X-ray fluorescence tomography. Our findings show two distinct types of crystal surfaces that persist during coccolith development: flat and curved, which differ also in roughness. Interestingly, even though both types of surfaces have variable degrees of membrane confinement, they are separated by distinct "bands" of intimate contact between the crystals and the membrane. We propose that these "bands" serve as boundaries, creating subcompartments within the coccolith vesicle that regulate crystal growth and its cessation. These results suggest that the coccolith vesicle membrane maintains distinct and functional chemical environments on the nanometer scale.