Biomineralization Within Vesicles: The Calcite of Coccoliths

212 Young & Henriksenbiomineralization systems, hence it is reasonable to infer that the nucleation controlmechanism should be similar for all coccolith cases. For the V/R alternation of nuclei inthe heterococcolith rim, template control has been a tempting theory, particularly sincethe nucleation is confined to single, well-defined belts with the organic base-plate scaleavailable as a substrate for the nucleation template (Young et al. 1992; Marsh et al.1999). However, with holococcoliths and heterococcolith central area structures,crystallographic orientation control is equally precise, but nucleation is much moredispersed. Hence, a self-organizing system seems more likely than nucleation on a predesignedtemplate. Given this insight, in vitro experiments on the effects of coccolithassociatedmacromolecules become more important.Crystal growth regulationThe styles of growth regulation seen in heterococcoliths, holococcoliths andnannoliths are rather variable. Heterococcolith rim crystals show selected growth inparticular directions, complex development of rhombic faces to fill non-rhombohedralspace and blocked growth to produce non-crystalline surfaces. Heterococcolith centralarea growth tends to show only rhombic face development, but with elaborateinterlocking crystal growth. In holococcoliths, growth is confined to rhombohedra butblocked at a finite size. So there is great variation in the details of precise regulationwithin a basic motif.Genomic approachesThe parallel approaches of biochemical and morphological research have been appliedrather extensively to coccolithophore calcification and are arguably at a relatively maturestate of knowledge, although new insights are emerging from atomic force microscopy(Henriksen et al. in press a,b) and from study of polysaccharide effects in mutant strains(Marsh and Dickinson 1997; Marsh 2000). Probably the most promising approach for thefuture will be genomic research aimed at identifying the suite of genes responsible forbiomineralization and their functions. In this general field, the coccolithophores mayprove to be ideal test cases for a number of reasons. First, as protests, they have relativelysmall genomes. Second, they are readily grown in culture and can be experimented uponwithout ethical dilemmas. Third, the considerable interest in coccolithophores from otherperspectives means that genomic projects on them are already underway. Finally, the factthat different biomineralization modes occur within two phases of a single organismprovides a natural experimental system for comparing the biomineralization-related genesexpressed in the two life stages.ACKNOWLEDGMENTSWe are grateful to many colleagues for scientific collaboration over an extendedperiod, especially Paul Bown, Markus Geisen, Steven Mann, Ian Probert, Blair Steel,Susan Stipp, Mary Marsh and Peter Westbroek. Research funding has been provided bythe UK Natural Environment Research Council, The European Union (through theCODENET FP4 TMR network project) and the Danish Research Council.REFERENCESAlcober J, Jordan RW (1997) An interesting association between Neosphaera coccolithomorpha andCeratolithus cristatus (Haptophyta). Eur J Phycol 32:91-93Billard C (1994) Life cycles. In: The Haptophyte Algae. Green JC, Leadbeater BSC (eds) Clarendon Press,Oxford, p 167-186Billard C, Inouye I (in press) What's new in coccolithophore biology? In: Coccolithophores - Frommolecular processes to global impact. Thierstein HR, Young JR (eds) Springer, Heidelberg