Chemistry Reference
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record, which shows coccoliths first occurring in the Late Triassic (Bown 1998a,b) ca.
220 Ma. Calcification in coccolithophores occurs primarily in Golgi-derived vesicles.
Since Golgi bodies are a primitive feature of eukaryotes, this makes an endosymbiotic
origin of calcification extremely unlikely. So coccolith calcification has probably evolved
de novo
relatively recently and entirely independently from biomineralization in other
groups. It may thus prove a useful outlier group for testing hypotheses of fundamental
similarities in biomineralization mechanisms among groups that adopted
biomineralization in the Cambrian radiation (Kirschvink and Hagadorn 2000).
Coccolith biomineralization has been rather extensively researched; reviews include
de Vrind-de Jong et al. (1986, 1994), Westbroek et al. (1989), Pienaar (1994), de Vrind-
de Jong and de Vrind (1997), Young et al. (1999), and Marsh (2000). This interest
reflects both the importance of coccolithophores and the fact that, as rapidly reproducing
protists, they are amenable to laboratory culture and study. At least some of the pioneers
of research in this field also anticipated that, as unicellular organisms, the
coccolithophores would show simpler biomineralization mechanisms than higher
organisms (Westbroek pers comm.). However, coccolith biomineralization has proven to
be under strong cellular control, lying at the biologically controlled end of the spectrum
of biomineralization mechanisms. This probably reflects the fact that mineralization
occurs within intracellular vesicles. This complex biomineralization process is still only
partially characterized. The majority of research on coccolith biomineralization has come
from detailed laboratory study of a very limited range of taxa, including cytological and
biochemical aspects. These findings are briefly reviewed here, but the main focus is on
morphological, crystallographic and structural aspects of coccoliths from across the
spectrum of coccolith diversity. This approach is adopted to constrain mechanistic
hypotheses and identify targets for future research.
Life cycles
One of the most distinctive aspects of coccolithophore biomineralization is that two
very different biomineralization modes typically occur within the life cycle of single
species. This is unusual and makes for interesting biomineralization research possibilities,
but has been rather neglected, since the life cycle details have only recently been well
established. Protist life cycles are often difficult to study, as they are heavily dependent
upon fortuitous observations of laboratory cultures. Haptophytes definitely follow this
pattern, as our knowledge of life cycles has long been confined to a few rather disparate
case studies. The most notable example is the coccolithophore
Coccolithus pelagicus,
which was shown by Parke and Adams (1960) to have a two phase life cycle, with both
phases producing coccoliths, but of very different types. Fresnel (1986) and Billard
(1994) synthesized these observations and hypothesized that a common pattern could be
observed, based on a haplo-diploid life cycle. Subsequently, a steady accumulation of
new data (e.g.
,
Green, et al. 1996; Cros et al. 2000; Geisen et al. 2002; Noel et al. in
press) supported this inference, although without direct testing of the key hypothesis that
coccolith type was directly related to ploidy level. More recently, life cycle transitions of
this type have occurred in laboratory cultures of three additional species from families
spanning the evolutionary diversity of coccolithophores. This allowed critical testing of
the ploidy level hypothesis via flow cytometric measurement of DNA content (Houdan et
al. in press).
Following this work, a basic model of coccolithophore life cycles can now be given
(Fig. 1). As is typical for eukaryotes, the fundamental aspect is alternation of diploid and
haploid phases; diploid cells have two copies of each chromosome, and so possess 2N
chromosomes, while haploid cells have only one copy, and so have N chromosomes.
Transition from the diploid to haploid phase occurs through meiosis, which involves cell
