Ultrastructure and crystallography of nanoscale calcite building blocks in Rhabdosphaera clavigera coccolith spines

Coccolithophores create an intricate exoskeleton from nanoscale calcite platelets. Shape, size, and crystal orientation are controlled to a remarkable degree. In this study, the structure of Rhabdosphaera clavigera is described in detail for the first time through a combination of electron microscopy techniques, including three-dimensional electron tomography. The coccolithophore exhibits several micrometer long 5-fold symmetric spines with diameters of approximately 0.5 μm. The nanorystals constituting the spine are arranged radially along the longitudinal axis, protruding from the almost flat disks that form the coccosphere. The stem of the spine is shown to consist of {104} calcite rhombohedra single crystalline platelets, arranged in five separate spiral "staircases". The spine tip shows 15 structural elements: five large "panels" protruding outward along the lateral plane and five leaf-shaped smaller units which form the topmost steps of the staircases. The outer tip consists of five long thin platelets protruding along the length of the spine axis. This feature extends downward into the spine-core. This core-feature may serve as a base for crystal nucleation and assembly analogous to the proto-coccolith ring in the V/R growth model (Young J. R. et al. Nature, 1992, 356, 516-518). However, we find significant dissimilarities of the crystal elements constituting the spine in comparison to that model. © 2014 American Chemical Society.