Construction and morphogenesis of the chiral ultrastructure of coccoliths from the marine alga Emiliania huxleyi

Abstract

Electron microscopy was used to study the developmental stages in coccolith biomineralization for the marine alga Emiliania huxleyi, with particular reference to the formation of the chiral ultrastructure. Transmission electron microscopy showed that the earliest stages in coccolith formation are characterized by an elliptical proto-coccolith ring of discrete single crystals of calcite. These initial crystals appear to be deposited with alternating radial (R) and vertical (V) orientations (the V/R nucleation model). Scanning electron microscopy showed that the R-crystals initially develop in the vertical direction to form rhombic plates, possibly with rhombohedral {$\overline{1}$018} faces and a Z-shaped cross section. When viewed from above the cell, further growth involves an anticlockwise tangential extension from each R-crystal which overlaps the neighbouring R-crystal on the inner rim and generates the morphological handedness. The result is an apparent inner and outer tube cycle of R-crystals with opposite imbrication. Subsequent growth of the plate-like extension occurs radially inwards, towards the foci of the elliptical ring, whereas the external surface develops outwards, in a direction perpendicular to the {$\overline{1}$018} face, to form the shield elements. Tracings of elements for mature coccoliths revealed a c-axis distribution that was best modelled by an offset of this axis by approximately 20 degrees to the normal for the local tangent. The chiral ultrastructure suggests that the calcite crystals are enclosed within vesicles which are themselves laid down in a chiral arrangement. Implications for the V/R model are discussed.