In oligotrophic surface waters, typical concentrations of coccolithophores are in the range 5000–
50 000 cells per liter. To put this in context, a
teaspoonful (5 ml) of typical surface open ocean
seawater will contain between 25 and 250 coccolithophore cells. Blooms of E. huxleyi have been
defined as concentrations exceeding 1 million cells
per liter; the densest bloom ever recorded, in
a Norwegian fiord, had a concentration of
115 000 000 cells per liter. Blooms of E. huxleyi
can cover large areas; the largest ever recorded
bloom occurred in June 1998 (see Figure 3) in
the North Atlantic south of Iceland and covered
about 1 million km
2
, 4 times the area of the United
Kingdom.
Coccoliths
Coccolithophores, and the coccolith shields with
which they surround themselves, are incredibly
small. And yet, despite their small size, coccoliths are
elegant and ornate structures, which, if the water
chemistry is suitable, are produced reliably with few
malformations. This efficient manufacture occurs at
a miniature scale: the diameter of an E. huxleyi
coccolith ‘spoke’ (Figure 1(c)) is of the order 50 nm,
considerably smaller than the wavelength range of
visible light (400–700 nm). Calcite is mostly transparent to visible light (unsurprisingly, given that
coccolithophores are photosynthetic) and the small
coccoliths are often at the limit of discrimination,
even under high magnification. However, under
cross-polarized light, coccoliths produce distinctive
patterns which are closely related to their structure.
As a result most coccoliths can be accurately identified by light microscopy. However, the details and
beauty of coccoliths can only be properly appreciated using electron microscopy (Figure 1).
Coccolithophores synthesize different types of
coccoliths during different life-cycle stages. Here we
concentrate on the heterococcoliths associated with
the diploid life stage. These heterococcoliths are
formed from crystal units with complex shapes, in
contrast to holococcoliths which are constructed out
of smaller and simpler crystal constituents. Coccoliths are typically synthesized intracellularly (within
a vesicle), probably one at a time, and subsequently
extruded to the cell surface. The time taken to form a
single coccolith can be less than 1 h for E. huxleyi.
Coccoliths continue to be produced until a complete
coccosphere covering (made up of maybe 20 coccoliths, depending on species) is produced.
Most coccolithophores construct only as many
coccoliths as are required in order to provide a
complete single layer around their cell. Emiliania
huxleyi is unusual in that, under certain conditions,
it overproduces coccoliths; many more coccoliths are
built than are needed to cover the cell. In these
conditions, multiple layers of coccoliths accumulate
around the E. huxleyi cell until the excessive covering eventually becomes unstable and some of the
coccoliths slough off to drift free in the water. The
large number of unattached coccoliths accompanying an E. huxleyi bloom contributes to a great extent
to the turbidity of the water and to the perturbations
to optics that make the blooms so apparent from
space.
Curiously, the functions of coccoliths are still uncertain. It is probable that a major function is to
provide some protection from grazing by zooplankton, but many alternative hypotheses have also
been advanced. For instance, the coccoliths may increase the rate of sinking of the cells through the
water (and therefore also enhance the rate at which
nutrient-containing water flows past the cell surface)
or they may provide protection against the entry of
viruses or bacteria to the cell. At one time it was
thought that coccoliths might provide protection
against very high light intensities, which could explain the resistance to photoinhibition apparent in
E. huxleyi, but various experimental results make
this explanation unlikely. One species, F. profunda, a
member of the deeper ‘shade flora’, orients its coccoliths in such a way that they conceivably act as a
light-focusing apparatus maximizing photon capture
in the darker waters it inhabits (Figure 1(e)). Some
species produce trumpet-like protrusions from each
coccolith (Figure 1(f)), again for an unknown purpose. Currently there is a paucity of hard data with
which to discriminate between the various hypotheses for coccolith function, and the diversity of coccolith morphology makes it likely that they have
been adapted to perform a range of functions.
Life Cycle
Many details are still obscure, and data are only
available from a limited number of species, but it
appears that most coccolithophores alternate between fully armored (heterococcolith-covered) diploid life stages and less-well-armoured (either
holococcolith-covered or else naked) haploid phases.
Both phases are capable of indefinite asexual reproduction, which is rather unusual among protists.
That sexual reproduction also occurs fairly frequently is evidenced by the observation of significant
genetic diversity within coccolithophore blooms.
Bloom populations do not consist of just one clone
COCCOLITHOPHORES 405
50 000 cells per liter. To put this in context, a
teaspoonful (5 ml) of typical surface open ocean
seawater will contain between 25 and 250 coccolithophore cells. Blooms of E. huxleyi have been
defined as concentrations exceeding 1 million cells
per liter; the densest bloom ever recorded, in
a Norwegian fiord, had a concentration of
115 000 000 cells per liter. Blooms of E. huxleyi
can cover large areas; the largest ever recorded
bloom occurred in June 1998 (see Figure 3) in
the North Atlantic south of Iceland and covered
about 1 million km
2
, 4 times the area of the United
Kingdom.
Coccoliths
Coccolithophores, and the coccolith shields with
which they surround themselves, are incredibly
small. And yet, despite their small size, coccoliths are
elegant and ornate structures, which, if the water
chemistry is suitable, are produced reliably with few
malformations. This efficient manufacture occurs at
a miniature scale: the diameter of an E. huxleyi
coccolith ‘spoke’ (Figure 1(c)) is of the order 50 nm,
considerably smaller than the wavelength range of
visible light (400–700 nm). Calcite is mostly transparent to visible light (unsurprisingly, given that
coccolithophores are photosynthetic) and the small
coccoliths are often at the limit of discrimination,
even under high magnification. However, under
cross-polarized light, coccoliths produce distinctive
patterns which are closely related to their structure.
As a result most coccoliths can be accurately identified by light microscopy. However, the details and
beauty of coccoliths can only be properly appreciated using electron microscopy (Figure 1).
Coccolithophores synthesize different types of
coccoliths during different life-cycle stages. Here we
concentrate on the heterococcoliths associated with
the diploid life stage. These heterococcoliths are
formed from crystal units with complex shapes, in
contrast to holococcoliths which are constructed out
of smaller and simpler crystal constituents. Coccoliths are typically synthesized intracellularly (within
a vesicle), probably one at a time, and subsequently
extruded to the cell surface. The time taken to form a
single coccolith can be less than 1 h for E. huxleyi.
Coccoliths continue to be produced until a complete
coccosphere covering (made up of maybe 20 coccoliths, depending on species) is produced.
Most coccolithophores construct only as many
coccoliths as are required in order to provide a
complete single layer around their cell. Emiliania
huxleyi is unusual in that, under certain conditions,
it overproduces coccoliths; many more coccoliths are
built than are needed to cover the cell. In these
conditions, multiple layers of coccoliths accumulate
around the E. huxleyi cell until the excessive covering eventually becomes unstable and some of the
coccoliths slough off to drift free in the water. The
large number of unattached coccoliths accompanying an E. huxleyi bloom contributes to a great extent
to the turbidity of the water and to the perturbations
to optics that make the blooms so apparent from
space.
Curiously, the functions of coccoliths are still uncertain. It is probable that a major function is to
provide some protection from grazing by zooplankton, but many alternative hypotheses have also
been advanced. For instance, the coccoliths may increase the rate of sinking of the cells through the
water (and therefore also enhance the rate at which
nutrient-containing water flows past the cell surface)
or they may provide protection against the entry of
viruses or bacteria to the cell. At one time it was
thought that coccoliths might provide protection
against very high light intensities, which could explain the resistance to photoinhibition apparent in
E. huxleyi, but various experimental results make
this explanation unlikely. One species, F. profunda, a
member of the deeper ‘shade flora’, orients its coccoliths in such a way that they conceivably act as a
light-focusing apparatus maximizing photon capture
in the darker waters it inhabits (Figure 1(e)). Some
species produce trumpet-like protrusions from each
coccolith (Figure 1(f)), again for an unknown purpose. Currently there is a paucity of hard data with
which to discriminate between the various hypotheses for coccolith function, and the diversity of coccolith morphology makes it likely that they have
been adapted to perform a range of functions.
Life Cycle
Many details are still obscure, and data are only
available from a limited number of species, but it
appears that most coccolithophores alternate between fully armored (heterococcolith-covered) diploid life stages and less-well-armoured (either
holococcolith-covered or else naked) haploid phases.
Both phases are capable of indefinite asexual reproduction, which is rather unusual among protists.
That sexual reproduction also occurs fairly frequently is evidenced by the observation of significant
genetic diversity within coccolithophore blooms.
Bloom populations do not consist of just one clone
COCCOLITHOPHORES 405