CONSTRUCTION AND MORPHOGENESIS OF THE CHIRAL ULTRASTRUCTURE OF COCCOLITHS FROM THE MARINE ALGA EMILIANIA-HUXLEYI

Electron microscopy was used to study the developmental stages in cocc olith biomineralization for the marine alga Emiliania huxleyi, with pa rticular reference to the formation of the chiral ultrastructure. Tran smission electron microscopy showed that the earliest stages in coccol ith 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) orientati ons (the V/R nucleation model). Scanning electron microscopy showed th at the R-crystals initially develop in the vertical direction to form rhombic plates, possibly with rhombohedral {($) over bar 1018} faces a nd a Z-shaped cross section. When viewed from above the cell, further growth involves an anticlockwise tangential extension from each R-crys tal which overlaps the neighbouring R-crystal on the inner rim and gen erates the morphological handedness. The result is an apparent inner a nd outer tube cycle of R-crystals with opposite imbrication. Subsequen t growth of the plate-like extension occurs radially inwards, towards the foci of the elliptical ring, whereas the external surface develops outwards, in a direction perpendicular to the {($) over bar 1018} fac e, to form the shield elements. Tracings of elements for mature coccol iths revealed a c-axis distribution that was best modelled by an offse t 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 chir al arrangement. Implications for the V/R model are discussed.