317
In recent review of the carbon physiology of coccolithophores Sikes and Fabry (1990)
concluded that increases in the productivity of these organisms were unlikely to cause
significant outgassing of CO 2 due mainly to the buffering capacity of seawater, but would
maintain a downward flux of particulate carbon in the oceans. A more critical question
concerns the difference in CO 2 exchange between ecosystems dominated by coccolithophores
as opposed to non-calcifying phytoplankton (e.g. diatoms). A model of primary production
in the North Atlantic (Taylor et al., 1991) shows that, in the summer period of nutrient
limited conditions, the presence of coccolithophores raises the partial pressure of CO 2 at the
surface, thereby reducing the atmosphere-to-ocean flux of this gas while maintaining a higher
downward flux of particulate (organic + inorganic) carbon due to the formation of calcite.
This flux is balanced by the upward mixing of dissolved inorganic carbon from beneath the
seasonal thermocline. The net result over a longer period of time is a change in the CO 2
chemistry of surface water (lower alkalinity and TC02) accompanied by higher atmospheric
CO 2 which is the same conclusion reached by the geochemical mass budget models (Berger,
1982; Dymond and Lyle, 1985).
SULPHUR CYCLING
The recognition that the ocean is a strong source of biogenic sulphur gases which, on
oxidation in the atmosphere, play an important role in the formation of cloud condensation
nuclei (CCN) (Charlson et ai., 1987) and contribute to the acidity of rainfall has led to
considerable efforts to characterise processes that may cause variations in the gas fluxes. It
is now well known (Andreae, 1986) that the ocean emission is mainly in the form of
dimethyl sulphide (DMS) formed by the cleavage of dimethylsulphoniopropionate (DMSP), an
important osmosolute in certain groups of phytoplankton (Dickson and Kirst, 1987).
Measurements for natural water samples and cultures have demonstrated that coccolithophores
contain large quantities of DMSP at internal concentrations consistent with an osmoregulatory
function, and are associated with high levels of DMS in the water (Turner et aI., 1988; Keller
et ai., 1989). However, the proportion of the total DMS flux from the oceans attributable to
this group of phytoplankton remains uncertain.
Levels of algal (as opposed to free) DMSP measured in the surface waters of the NE Atlantic
in summer 1987 were high and closely correlated with phytoplankton biomass within
coccolithophore blooms (Fig. 9). By contrast, quantities of free DMS were extremely variable
confirming that the processes controlling the conversion of DMSP to DMS are important
determinants of the proportion of DMSP that reaches the atmosphere as DMS. These include
grazing (Dacey and Wakeham, 1986), and microbial activity which affects both the
In recent review of the carbon physiology of coccolithophores Sikes and Fabry (1990)
concluded that increases in the productivity of these organisms were unlikely to cause
significant outgassing of CO 2 due mainly to the buffering capacity of seawater, but would
maintain a downward flux of particulate carbon in the oceans. A more critical question
concerns the difference in CO 2 exchange between ecosystems dominated by coccolithophores
as opposed to non-calcifying phytoplankton (e.g. diatoms). A model of primary production
in the North Atlantic (Taylor et al., 1991) shows that, in the summer period of nutrient
limited conditions, the presence of coccolithophores raises the partial pressure of CO 2 at the
surface, thereby reducing the atmosphere-to-ocean flux of this gas while maintaining a higher
downward flux of particulate (organic + inorganic) carbon due to the formation of calcite.
This flux is balanced by the upward mixing of dissolved inorganic carbon from beneath the
seasonal thermocline. The net result over a longer period of time is a change in the CO 2
chemistry of surface water (lower alkalinity and TC02) accompanied by higher atmospheric
CO 2 which is the same conclusion reached by the geochemical mass budget models (Berger,
1982; Dymond and Lyle, 1985).
SULPHUR CYCLING
The recognition that the ocean is a strong source of biogenic sulphur gases which, on
oxidation in the atmosphere, play an important role in the formation of cloud condensation
nuclei (CCN) (Charlson et ai., 1987) and contribute to the acidity of rainfall has led to
considerable efforts to characterise processes that may cause variations in the gas fluxes. It
is now well known (Andreae, 1986) that the ocean emission is mainly in the form of
dimethyl sulphide (DMS) formed by the cleavage of dimethylsulphoniopropionate (DMSP), an
important osmosolute in certain groups of phytoplankton (Dickson and Kirst, 1987).
Measurements for natural water samples and cultures have demonstrated that coccolithophores
contain large quantities of DMSP at internal concentrations consistent with an osmoregulatory
function, and are associated with high levels of DMS in the water (Turner et aI., 1988; Keller
et ai., 1989). However, the proportion of the total DMS flux from the oceans attributable to
this group of phytoplankton remains uncertain.
Levels of algal (as opposed to free) DMSP measured in the surface waters of the NE Atlantic
in summer 1987 were high and closely correlated with phytoplankton biomass within
coccolithophore blooms (Fig. 9). By contrast, quantities of free DMS were extremely variable
confirming that the processes controlling the conversion of DMSP to DMS are important
determinants of the proportion of DMSP that reaches the atmosphere as DMS. These include
grazing (Dacey and Wakeham, 1986), and microbial activity which affects both the