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an unequivocal physio-ecological explanation for coccolith formation. From a geological
perspective, calcification in marine organisms is generally considered to be a biotic response
to calcium toxicity which evolved some 600 million years ago at a time of rapidly increasing
calcium concentrations in the sea (Degens and Ittekot, 1986). Biochemical investigations of
algae indicate that precipitation of CaC03 is associated with increases in alkalinity and Ph
during the photosynthetic assimilation of CO2 and HC03- (Borowitzka, 1987). As suggested
by the experiments of Baumann et al. (1978), calcification may be a means of getting rid of
excess photosynthetic energy under conditions of nutrient limitation of cell growth. Various
other hypotheses based on possible morphological functions of coccoliths, including light
shading and cell sinking, have also been put forward.
The biogeochemical significance of the coccolithophores concerns, in particular, the global
carbon and sulphur cycles. Calcium carbonate is the major form in which carbon is buried in
the sea, with the most important groups of contributing organisms being the coccolithophores
(Bramlette, 1958) and foraminifera in the oceans, and corals, benthic algae and molluscs in
shallow waters. The present rate of marine carbonate-carbon burial is not well known
(Milliman and Takahashi, 1990), but is probably at least twice that for organic carbon which
is preserved mainly in shallow water, deltaic sediments (Berner, 1982). Coccolithophores also
contribute to the organic carbon content of ocean sediments through the synthesis of refractory
compounds such as long-chained ketones (Brassell et al., 1986), and indirectly through the
adsorption of organic matter onto calcite surfaces (Suess, 1973).
Over geological timescales, periods of high rates of carbonate burial (Chamberlin, 1898) and
of abundant coccolithophores (e.g. Roth, 1987) are associated with warmer climates (and,
presumably, elevated levels of atmospheric CO2). It has been postulated that increases in deep
ocean carbonate burial by coccolithophores will tend to cause a relative warming of the earth
(Volk, 1989) through effects on the geochemical carbonate-silicate cycle (Berner and Lasaga,
1989) which lead to an increase in levels of atmospheric CO2• However, the potential
influence of short term variations in coccolithophore productivity on atmospheric CO 2 and on
global temperature, for example over glacial-interglacial cycles, is not well understood.
In the case of sulphur, coccolithophores are known to be a major oceanic source of
dimethyl sulphide (DMS) (Andreae, 1986; Turner et al., 1988) which is thought to affect the
formation of cloud condensation nuclei in the marine atmosphere (Charlson et al., 1987). This
problem is of climatological significance because of the important role of clouds in the
regulation of global temperature (Slingo, 1990).
an unequivocal physio-ecological explanation for coccolith formation. From a geological
perspective, calcification in marine organisms is generally considered to be a biotic response
to calcium toxicity which evolved some 600 million years ago at a time of rapidly increasing
calcium concentrations in the sea (Degens and Ittekot, 1986). Biochemical investigations of
algae indicate that precipitation of CaC03 is associated with increases in alkalinity and Ph
during the photosynthetic assimilation of CO2 and HC03- (Borowitzka, 1987). As suggested
by the experiments of Baumann et al. (1978), calcification may be a means of getting rid of
excess photosynthetic energy under conditions of nutrient limitation of cell growth. Various
other hypotheses based on possible morphological functions of coccoliths, including light
shading and cell sinking, have also been put forward.
The biogeochemical significance of the coccolithophores concerns, in particular, the global
carbon and sulphur cycles. Calcium carbonate is the major form in which carbon is buried in
the sea, with the most important groups of contributing organisms being the coccolithophores
(Bramlette, 1958) and foraminifera in the oceans, and corals, benthic algae and molluscs in
shallow waters. The present rate of marine carbonate-carbon burial is not well known
(Milliman and Takahashi, 1990), but is probably at least twice that for organic carbon which
is preserved mainly in shallow water, deltaic sediments (Berner, 1982). Coccolithophores also
contribute to the organic carbon content of ocean sediments through the synthesis of refractory
compounds such as long-chained ketones (Brassell et al., 1986), and indirectly through the
adsorption of organic matter onto calcite surfaces (Suess, 1973).
Over geological timescales, periods of high rates of carbonate burial (Chamberlin, 1898) and
of abundant coccolithophores (e.g. Roth, 1987) are associated with warmer climates (and,
presumably, elevated levels of atmospheric CO2). It has been postulated that increases in deep
ocean carbonate burial by coccolithophores will tend to cause a relative warming of the earth
(Volk, 1989) through effects on the geochemical carbonate-silicate cycle (Berner and Lasaga,
1989) which lead to an increase in levels of atmospheric CO2• However, the potential
influence of short term variations in coccolithophore productivity on atmospheric CO 2 and on
global temperature, for example over glacial-interglacial cycles, is not well understood.
In the case of sulphur, coccolithophores are known to be a major oceanic source of
dimethyl sulphide (DMS) (Andreae, 1986; Turner et al., 1988) which is thought to affect the
formation of cloud condensation nuclei in the marine atmosphere (Charlson et al., 1987). This
problem is of climatological significance because of the important role of clouds in the
regulation of global temperature (Slingo, 1990).