Page 240 - Introduction to Paleobiology and The Fossil Record

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Box 9.10 Atomic force microscopy of coccolithophores

Coccolithophores, despite their small size, are attractive and sophisticated organisms. A number of
plate morphs, emphasizing the diversity of form within the group, have been described (Fig. 9.18c):
asterolith, star-shaped plates; cyclolith, open rings; lopadolith, vase-shaped morphs with elevated
edges; placolith, two disks fused by the median tube; stetolith, column-shaped plates; zygolith, ellipti-
cal ring with arches applied to holococcoliths. Apart from the term placolith most are not in routine
use. Additionally, helioliths, composed of a large number of small radially arranged crystals, and
ortholiths, with only a few crystals, have been recognized.
The coccolithophore is precipitated within the cell from the coccolith vesicle or Golgi body with
tightly regulated crystal growth, allowing the crystals to integrate as the complex and exquisite net-
works that comprise a complete skeleton. Karen Henriksen, a former graduate student at the Uni-
versity of Copenhagen, applied atomic force microscopy (AFM) to the surface of three coccolith
species, a technique that allows investigation at higher orders of magnitude than even scanning
electron microscopy (SEM) and transmission electron microscopy (TEM) equipment. Henriksen and
colleagues (2004) established key differences among these taxa suggesting that subtle changes in the

mechanisms of biomineralization can drive significant changes in morphology that have knock-on
effects for the adaptability, lifestyle and distribution of the coccolith species. The large morphological
disparity seen in this remarkable group is thus a function of the mode and orientation of crystal
growth at the atomic level and where the organism ultimately lived depended on the whims of a
crystal lattice.

they then migrate to the cell surface and are flagellate, and is usually coated by minute
expelled to form a composite exoskeleton, the holococcoliths; the diploid phase (with full
coccosphere. Commonly the coccosphere complement of chromosomes) is usually non-
consists of 10–30 discrete coccoliths, although flagellate, and is coated by heterococcoliths.

some forms have many more (Box 9.10). Both phases are capable of indefi nite asexual
Many taxa produce coccospheres formed of reproduction and it appears likely that the
only one type of coccolith, but others show a two-phase life cycle is an adaptation allowing
variety of coccolith morphologies (Fig. 9.18); coccolithophores to survive challenging eco-
in particular there are often specialized coc- logical conditions. The haploid (holoccolith-
coliths around the flagellar pole of the cell. producing) phase is thought to be adapted to

There are two fundamentally different types oligotrophic conditions (when nutrients are
of coccoliths: heterococcoliths have a radial scarce) whilst the diploid (heterococcolith-
array of relatively few (typically 20–50) producing) phase is thought to be adapted to
complex-shaped crystal units, whereas holo- more eutrophic conditions (when nutrients
coccoliths are formed of planar arrays of hun- are abundant).

dreds of minute uniform-sized (typically c. The classification of extant coccolitho-
0.1 μm) rhombohedral crystallites. phores is based largely on coccosphere mor-
Haptophyte life cycles were very poorly phology and coccolith structure because the
known until recently; research has now shown intricate and distinctive form of coccoliths
that cocolithophores, and possibly most hap- makes them ideal for morphological classifi -
tophtes, typically have alternating haploid cation. Cell characters can only be studied
and diploid stages that are both capable with transmission electron microscopy and
of asexual reproduction. Coccolithophores have generally proved rather invariant. Data
usually have life cycles consisting of two main from cytology and molecular genetics have
phases producing radically different cocco- strongly supported the classifi cation based on
liths that were often described initially as two morphological criteria. The reliance on coc-
different species. The haploid phase (with half coliths in the extant classifi cation also means
the complement of chromosomes) is always that there are relatively few problems in align-