haptophytes. These alkenones tend to survive degradation in sediments, and the ratio of one type of
alkenones to another (the U
37
K index) can be used to
estimate past ocean temperatures. Calcareous nannofossils are also extremely useful in determining the
age of different layers in cores of ocean sediments
(biostratigraphy).
The Future
The pH of the oceans is falling (they are becoming
increasingly acidic), because of the invasion of fossil
fuel-derived CO 2 into the oceans. Surface ocean pH
has already dropped by 0.1 units and may eventually
drop by as much as 0.7 units, compared to preindustrial times, depending on future CO 2 emissions.
The distribution of dissolved inorganic carbon (DIC)
between bicarbonate, carbonate, and dissolved CO 2
gas changes with pH in such a way that carbonate ion
concentration (and therefore saturation state, O) is
decreasing even as DIC is increasing due to the invading anthropogenic CO 2 . It is predicted that, by the
end of this century, carbonate ion concentration and
O may have fallen to as little as 50% of preindustrial
values. If emissions continue for decades and centuries
without regulation then the surface oceans will eventually become undersaturated with respect to calcium
carbonate, first with respect to the more soluble aragonite used by corals, and some time later also with
respect to the calcite formed by coccolithophores.
There has been an increasing appreciation over the
last few years that declining saturation states may
well have significant impacts on marine life, and, in
particular, on marine organisms that synthesize
CaCO 3 . Experiments on different classes of marine
calcifiers (CaCO 3 synthesizers) have demonstrated a
reduction in calcification rate in high CO 2 seawater.
One such experiment showed a strong decline in
coccolithophore calcification rate (and a notable increase in the numbers of malformed coccoliths) at
high CO 2 (low saturation state), although some
other experiments have obtained different results. If
coccolithophore biocalcification is controlled by O
then the explanation could be linked to the importance of O in controlling inorganic calcification, although coccolithophores calcify intracellularly and
so such a link is not guaranteed. At the time of
writing, further research is being undertaken to determine whether, as the oceans become more acidic,
coccolithophores will continue to be able to synthesize coccoliths and subsequently maintain them
against dissolution.
Our ability to predict the consequences of ocean
acidification on coccolithophores is hampered by our
poor understanding of the function of coccoliths
(what they are for, and therefore how the cells will be
affected by their absence), and also by our poor
understanding of the possibilities for evolutionary
adaptation to a low-pH ocean. These constraints can
be overcome to an extent by examining the geological
history of coccolithophores, and their (in)ability to
survive previous acid ocean events in Earth history.
Although coccoliths (and other calcareous nannofossils) have been widely studied by geologists, it is
only recently that there has been a concerted effort to
study their species turnover through events in Earth
history when the oceans were more acidic than now.
Although many authors have taken the success of
coccolithophores during the high-CO 2 late Cretaceous as reassuring with respect to their future prospects, the reasoning is fallacious. Levels of calcium
are thought to have been higher than now during the
Cretaceous, and the CCD (the depth at which CaCO 3
disappears from sediments due to dissolution, which
is a function of deep-water O) was only slightly
shallower than today, indicating that Cretaceous
seawater conditions were not analogous to those to be
expected in a future high-CO 2 world.
It turns out that coccolithophores survived
the Paleocene–Eocene Thermal Maximum event
(thought to more closely resemble the predicted future) fairly well, with a modest increase in extinction
rates matched by a similar increase in speciation
rates. On the other hand, the environmental changes
at the Cretaceous–Tertiary boundary (the K/T impact
event), which also appears to have induced acidification, led to a mass extinction of 93% of all coccolithophore species, as well as to extinction of many
other calcifying marine organisms including ammonites. It is necessary to more accurately characterize the environmental changes that took place
across such events, in order to better determine how
well they correspond to the ongoing and future ocean
acidification.
See also
Calcium Carbonates. Benthic Foraminifera.
Further Reading
Gibbs SJ, Bown PR, Sessa JA, Bralower TJ, and
Wilson PA (2007) Nannoplankton extinction and
origination across the Paleocene–Eocene Thermal
Maximum. Science 314: 1770--1773 (doi: 10.1126/
science.1133902).
Holligan PM, Fernandez E, Aiken J, et al. (1993) A
biogeochemical study of the coccolithophore Emiliania
COCCOLITHOPHORES 409
alkenones to another (the U
37
K index) can be used to
estimate past ocean temperatures. Calcareous nannofossils are also extremely useful in determining the
age of different layers in cores of ocean sediments
(biostratigraphy).
The Future
The pH of the oceans is falling (they are becoming
increasingly acidic), because of the invasion of fossil
fuel-derived CO 2 into the oceans. Surface ocean pH
has already dropped by 0.1 units and may eventually
drop by as much as 0.7 units, compared to preindustrial times, depending on future CO 2 emissions.
The distribution of dissolved inorganic carbon (DIC)
between bicarbonate, carbonate, and dissolved CO 2
gas changes with pH in such a way that carbonate ion
concentration (and therefore saturation state, O) is
decreasing even as DIC is increasing due to the invading anthropogenic CO 2 . It is predicted that, by the
end of this century, carbonate ion concentration and
O may have fallen to as little as 50% of preindustrial
values. If emissions continue for decades and centuries
without regulation then the surface oceans will eventually become undersaturated with respect to calcium
carbonate, first with respect to the more soluble aragonite used by corals, and some time later also with
respect to the calcite formed by coccolithophores.
There has been an increasing appreciation over the
last few years that declining saturation states may
well have significant impacts on marine life, and, in
particular, on marine organisms that synthesize
CaCO 3 . Experiments on different classes of marine
calcifiers (CaCO 3 synthesizers) have demonstrated a
reduction in calcification rate in high CO 2 seawater.
One such experiment showed a strong decline in
coccolithophore calcification rate (and a notable increase in the numbers of malformed coccoliths) at
high CO 2 (low saturation state), although some
other experiments have obtained different results. If
coccolithophore biocalcification is controlled by O
then the explanation could be linked to the importance of O in controlling inorganic calcification, although coccolithophores calcify intracellularly and
so such a link is not guaranteed. At the time of
writing, further research is being undertaken to determine whether, as the oceans become more acidic,
coccolithophores will continue to be able to synthesize coccoliths and subsequently maintain them
against dissolution.
Our ability to predict the consequences of ocean
acidification on coccolithophores is hampered by our
poor understanding of the function of coccoliths
(what they are for, and therefore how the cells will be
affected by their absence), and also by our poor
understanding of the possibilities for evolutionary
adaptation to a low-pH ocean. These constraints can
be overcome to an extent by examining the geological
history of coccolithophores, and their (in)ability to
survive previous acid ocean events in Earth history.
Although coccoliths (and other calcareous nannofossils) have been widely studied by geologists, it is
only recently that there has been a concerted effort to
study their species turnover through events in Earth
history when the oceans were more acidic than now.
Although many authors have taken the success of
coccolithophores during the high-CO 2 late Cretaceous as reassuring with respect to their future prospects, the reasoning is fallacious. Levels of calcium
are thought to have been higher than now during the
Cretaceous, and the CCD (the depth at which CaCO 3
disappears from sediments due to dissolution, which
is a function of deep-water O) was only slightly
shallower than today, indicating that Cretaceous
seawater conditions were not analogous to those to be
expected in a future high-CO 2 world.
It turns out that coccolithophores survived
the Paleocene–Eocene Thermal Maximum event
(thought to more closely resemble the predicted future) fairly well, with a modest increase in extinction
rates matched by a similar increase in speciation
rates. On the other hand, the environmental changes
at the Cretaceous–Tertiary boundary (the K/T impact
event), which also appears to have induced acidification, led to a mass extinction of 93% of all coccolithophore species, as well as to extinction of many
other calcifying marine organisms including ammonites. It is necessary to more accurately characterize the environmental changes that took place
across such events, in order to better determine how
well they correspond to the ongoing and future ocean
acidification.
See also
Calcium Carbonates. Benthic Foraminifera.
Further Reading
Gibbs SJ, Bown PR, Sessa JA, Bralower TJ, and
Wilson PA (2007) Nannoplankton extinction and
origination across the Paleocene–Eocene Thermal
Maximum. Science 314: 1770--1773 (doi: 10.1126/
science.1133902).
Holligan PM, Fernandez E, Aiken J, et al. (1993) A
biogeochemical study of the coccolithophore Emiliania
COCCOLITHOPHORES 409