Biomineralization Within Vesicles: The Calcite of Coccoliths

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210 Young & HenriksenFigure 11. Nannoliths. Two examples of species with life cycles involving heterococcoliths and nannolithsrather than heterococcoliths and holococcoliths. (A−C) Ceratolithus cristatus: (A) collapsed combinationcoccosphere with heterococcoliths and ceratolith nannolith; (B) coccosphere of heterococcoliths; (C)isolated ceratolith. (D−F) Alisphaera: (D) isolated nannoliths, minute cup-shaped aragonitic liths (alsoknown as Polycrater galapagensis); (E) coccosphere of the heterococcolith phase; (F) coccosphere of thenannolith (“Polycrater”) phase.
Biomineralization Within Vesicles 211are no direct observations on ploidy level (relative genome size) in the two phases. Wehypothesize that, as in all studied cases to date, the heterococcolith-producing phase willprove to be diploid. The nannoliths produce by these two genera are very different fromeach other as well as from either heterococcoliths or holococcoliths so they are veryunlikely to be directly related. Instead, we predict they will prove to be two additionalexamples of transfer of calcification from the diploid to the haploid phase with necessaryevolution of a new mechanism for calcification resulting each time in distinctivelydifferent biomineral structures.SummaryThese inferences can be summarized in a schematic evolutionary tree (Fig. 12).Heterococcolith biomineralization occurs consistently in the diploid phase and so is likelyto have been the primitive calcification mode. Subsequently calcification has apparentlybeen transferred to the haploid phase on at least three occasions with a differentbiomineralization mode evolving in each case. Given this, a comparison of featuresbetween the biomineralization modes may be of value.Crystallographic orientation—template control or a self-organizing system?Precise control of crystallographic orientation, including placement and spacing ofcrystal nuclei and both c- and a-axis orientation is a pervasive feature of coccolithbiomineralization shown equally well by heterococcolith rims, heterococcolith centralarea structures, nannoliths and holococcoliths. In general, this is not a common feature ofFigure 12. Evolution of biomineralization modes. Phylogenetic tree showing inferred pattern ofacquisition of biomineralization modes in haptophytes. The double lines represent the diploid (upper)and haploid (lower) life cycle phases, thick lines indicate biomineralization within these phases. Theposition of Ceratolithus and Alisphaera within the tree is speculative, the rest of the tree topologycorresponds to that from molecular genetic data (Edvardsen et al. 2000; Saez et al. in press).
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Biomineralization Within Vesicles: The Calcite of Coccoliths

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