Biomineralization Within Vesicles: The Calcite of Coccoliths

204 Young & HenriksenFigure 8. Growth pattern variations in Calcidiscus and Umbilicosphaera. Coccoliths of three closelyrelated species, in distal view. In each of these species the entire distal shield is formed of V-units, andthe visible surface are rhombic faces. (a) Calcidiscus leptoporus. (b) Umbilicosphaera sibogae. (c)well-formed Umbilicosphaera foliosa showing complex suture pattern and apparent division into twocycles of elements. (d) malformed U. foliosa showing a range of element morphologies from thosetypical of C. leptoporus to those typical of U. foliosa.The surfaces shown in Figures 8 and 9 are all characterized by growth across thesurface resulting in formation of inorganic rhombic crystal faces. These are common,especially on distal surfaces and on central area structures (e.g., Rhabdosphaera, Fig. 7).Another class of surfaces also occurs, characterized by absence of growth on the surfaceand development of non-crystal surfaces (Henriksen et al. in press b; Young et al. in press).The distal shield of Coccolithus pelagicus shows examples of both surface types; growthoccurs incrementally on the upper surface, which develops as a rhombic face, while nogrowth occurs on the lower surface, which develops as a trailing surface behind the growthfront. Some coccoliths, e.g., Emiliania huxleyi, appear to be almost entirely characterizedby this surface type, with growth confined to narrow fronts at the tip of the element. Withthis type of growth it seems likely that crystal growth is blocked by organic coatings, mostlikely of coccolith-associated polysaccharides (Young et al. 1999). Such coatings arevisible on TEM sections after staining for polysaccharides (e.g., Outka and Williams 1971;Marsh 1994; Klaveness 1976; Young et al. 1999). We have been able to demonstratethrough AFM dissolution studies that these coatings do protect the completed crystals(Henriksen et al. in press b). It is reasonable to infer that a coating of polysaccharide thatcan prevent dissolution of an element would also serve to inhibit crystal growth. Therefore,the difference between the two calcite surface types seen in coccoliths—rhombic calcite