Biomineralization Within Vesicles: The Calcite of Coccoliths

Biomineralization Within Vesicles 203micrometer to atomic scale and with these observations have established the preciserelationships between element morphology and crystal lattice (Fig. 5; Henriksen et al. inpress a). High resolution AFM of the flat surfaces of the elements (Fig. 5B and D) revealan atomic pattern identical to that known from rhombic {10¯14} faces of inorganic calcite(Stipp et al. 1994; Stipp 1999). It shows the characteristic surface unit cell, atomic rowsdefining the orientation of the a-axis vector, and a pairing of these rows that constrainsthe crystallographic orientation fully (Henriksen et al. in press a). C. pelagicus has{10¯14} faces on the whole distal shield surface and has element edges defined byrhombic cleavage (Fig. 5E), with individual elements stuck together by imbrication. Bycontrast, in O. fragilis the crystal lattice is rotated so that acute crystallographic cornersmake the complex element edges. This enables elements to interlock without overlap,allowing for very thin and delicate coccoliths (Fig. 5F). The outer part of the distal shieldconsists of rhombic faces that are broken by a series of steps forming the slope towardsthe middle, probably induced by constraints on growth space by the vesicle. Thus, in bothspecies the rhombic calcite motif is much in evidence, but detailed variations inorientation allow them to construct coccoliths with very different properties.An example of the importance of the interplay of rhombic growth directions withvesicle geometry is provided by comparing coccoliths of the closely related speciesCalcidiscus leptoporus, Umbilicosphaera foliosa and Umbilicosphaera sibogae (Fig. 8).These belong to the family Calcidiscaceae and have their distal shields entirely formedfrom V-units (Young 1998; Young et al. in press). Calcidiscus leptoporus (Fig. 8A)shows flat, overlapping elements with curved radial edges. The surfaces are almostcertainly rhombic calcite faces, as can be seen from overgrown specimens, fromexamination of face relationships in the central area, and by analogy with the relatedspecies Coccolithus pelagicus and Oolithotus fragilis. Umbilicosphaera sibogae (Fig.8B) is similar, but has a wider central opening and radial edges showing a pronouncedkink. Umbilicosphaera foliosa (Fig. 8C) also has flat, probably rhombic faces, but hastwo cycles in the shield. The inner cycle is imbricated clockwise, the outer is imbricatedcounter-clockwise, with both cycles showing crystallographic edges. Superficially, this isa very complex crystal structure. However, when a large number of specimens isinvestigated at high resolution, paying close attention to overgrown and malformedexamples, it becomes evident that both shield cycles are made up of the same crystalunits. This is illustrated in Figure 9. The crystal units actually have a rather simple basicshape, as indicated by the dotted lines on Figure 9B, which also reflect the shape of theelements on the proximal surface. This simplicity is in marked contrast to the apparentcomplexity on the distal surface, caused by counter-clockwise growth on the surface ofthe crystal in the outer part of the coccolith and clockwise growth in the inner part. Thedifference in growth direction is in fact a predictable result of the plane of the crystalsurface intersecting obliquely with a conical surface (Fig. 9C). This is not seen in C.leptoporus since the elements are less obliquely oriented, and there are fewer of them.Figure 8D shows a specimen of U. foliosa with rather irregular growth that has thecomplete range of element types, from simple elements like those typical of C.leptoporus, through kinked elements like those of U. sibogae, to the offset bicyclicelements typical of U. foliosa. The fact that all three can occur on one coccolith as aresult of malformation proves that no major changes in mechanism are necessary to formone rather than the other. Further, this suggests that the range of distal shield elementmorphologies seen in the three species arise as a product of interaction of crystalorientation and coccolith shape rather than through any direct control of crystal growth.This example is thought-provoking because it shows that in biomineralization, a trivialchange in process can result in a large morphological effect.