Biomineralization Within Vesicles: The Calcite of Coccoliths

Biomineralization Within Vesicles 193Figure 2. Coccolithogenesis in Pleurochrysis carterae. Schematic section, based on TEM images,through a coccolithophore showing coccolith formation within Golgi-derived vesicles. Reproduced withpermission from van der Wal et al. 1983.cisternae, derived from the Golgi body. The process commences with formation of anorganic scale. These scales have a distinctive microfibrillar ultrastructure that ischaracteristic of scales in the haptophytes, and homologous non-calcified scales occur inmost non-coccolithophorid haptophytes (e.g., Billard 1994). In some cases, the scales areexocytosed without further development, to form an inner layer of body scales. In others,the vesicle develops a rather complex form, with extensions containing dense particlestermed coccolithosomes. Nucleation of calcite then occurs around the periphery of thebase-plate scale followed by crystal growth upward and outward to form the completecoccolith. During coccolith growth, the vesicle gradually expands, remaining in ratherclose contact with the developing coccolith. After completion of the coccolith, the vesicledilates and at this stage, a dense organic coating is visible around and between thecoccolith crystals. Exocytosis then occurs, apparently by fusion of the vesicle membraneand cell membrane. In this species the coccolithosomes appear to play a key role in