Jm. Didymus et al., CONSTRUCTION AND MORPHOGENESIS OF THE CHIRAL ULTRASTRUCTURE OF COCCOLITHS FROM THE MARINE ALGA EMILIANIA-HUXLEYI, Proceedings - Royal Society. Biological Sciences, 258(1353), 1994, pp. 237-245
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.