Electron diffraction and high-resolution transmission electron microscopy indicate that the radial segments composing coccoliths of the marine alga Emiliania huxleyi are individual single crystalline units of calcite. The top and bottom surfaces of the plate-like lower element correspond to the (12̄10) and (1̄21̄0) faces, and the  direction (c axis) is oriented parallel to these faces and in the direction of elongation of the lower and upper elements. The [101̄0] direction is perpendicular to the direction of elongation and corresponds to the direction of growth of the hammer-head extension in the upper element. Each radial segment exhibits handedness when viewed from above. The peripheral edges of the lower element are truncated by (101̄4) and (1̄018) rhombohedral faces with the (1̄018) face being more extensively developed and always positioned to the left when viewed from above, indicating the specific growth of the (1̄21̄0) face on the surface of the underlying base plate. Energy-dispersive X-ray analysis (EDXA) showed no evidence for localized compositional changes in the coccolith components. A mechanism is proposed in which the biological control of the development of individual radial segments is determined by the initial orientation of calcite rhombohedra with the (1̄21̄0) face parallel to the organic base plate, followed by vectorial regulation of crystal growth in three dimensions through the spatial restrictions set by the developing ultrastructure of the surrounding coccolith vesicle membrane. The geometric, stereochemical and symmetry requirements of molecular interactions between ions in the (1̄21̄0) face and macromolecules in the polysaccharide-containing base plate are discussed.
|Journal||Proceedings of the Royal Society B: Biological Sciences|
|Publication status||Print publication - 22 Sep 1988|