Biomineralization Within Vesicles: The Calcite of Coccoliths

Biomineralization Within Vesicles 205Figure 9. Complex element morphology from simple crystal growth. Side view of an Umbilicosphaerafoliosa coccolith (top), detail of distal shield (middle), and interpretative diagram (bottom). Numberingon the crystal units indicates the degree of offset between the inner and outer cycles. The continuity ofthe crystal units between the cycles is directly visible on units 1 and 2 and is further indicated bydevelopment of parallel edges on the inner and outer cycles (lines on leading and trailing edges of units2 and 5). Dotted lines indicate summary trajectory of units 3 and 4; this is approximately the patternseen on the proximal surface of the distal shield. Diagram below: oblique section across the coccolithsurface illustrating how the differing senses of apparent imbrication in the inner and outer cycles can beproduced by a single set of crystal faces.faces and non-crystalline faces—may be a function of selective blocking bypolysaccharides. Nonetheless, is not clear why these two crystal surface types occur or howvesicles are able to block growth on some surfaces, but not others.HOLOCOCCOLITHSWhile heterococcoliths have been rather extensively studied, the alternativebiomineral form, holococcoliths (Fig. 10), formed in the haploid life cycle stage, hasreceived less attention and proven more enigmatic.