268
corals precipitate large amounts of dissolved carbonate, a process in which the symbiosis with
zooxanthellae is instrumental, and are therefore quite effective in sequestering carbon, directly
in the euphotic layer, despite the widely held view that their "net community production" (Le.
their capacity to export organic carbon) is low (see Mann, 1982). According to Be et al.
(1982), the symbiotic relationship with zooxanthellae allows corals and foraminifers (see
below) to be the foremost producers of biogenic calcite in the circumglobal warm-water belt,
the former over the shallow platforms and the latter over the deep sea bed. The possible
effects of calcite precipitation by coral reefs and other organisms on atmospheric C~ is
discussed at the end of the next section. In addition, coral reefs are thought to be net exporters
of nitrate and could be indirectly fuelling the planktonic carbon pump in surrounding
oligotrophic waters.
Denitrification is the antagonistic process to nitrogen fixation, by which oxidized forms of
combined nitrogen (nitrate, nitrite and nitrous oxide) are reduced to free nitrogen gas. As in
the case of nitrogen fixation, it is generally considered that denitrification only occurs under
anaerobic or low-oxygen conditions (see Codispoti, 1989). In the open ocean, sites favourable
for denitrification would be oxygen-deficient portions of the water column, below the euphotic
layer (e.g. Ward and Zafiriou, 1988). More generally, denitrification could take place in
reducing environments (e.g. marine snow) similar to those potentially favouring N2 fixation
(see above). According to Codispoti (1989), denitrification and nitrogen fixation may be
approximately balanced on a time scale of about 107 years, but imbalances in which
denitrification may exceed nitrogen fixation could reduce export production, and thus potential
sequestration of carbon, by 20 to 30% for periods of several thousands years.
PLANKTON ORGANISMS WITH HIGH CARBON CONTENT
Estimates of carbon export from measurements of nitrogen-derived new production are
generally based upon the so-called "Redfield ratio" (e.g. Redfield, 1958; Redfield et al.,
1963), i.e. in terms of atoms C/N/P = 106116/1 (hence C/N = 6.6), which corresponds to
the average cellular composition of marine plankton 6 • However, many plankton organisms
export an extra share of carbon, either as organic thecae or in the form of calcareous skeletal
6All elemental ratios will henceforth be quoted in atoms.
corals precipitate large amounts of dissolved carbonate, a process in which the symbiosis with
zooxanthellae is instrumental, and are therefore quite effective in sequestering carbon, directly
in the euphotic layer, despite the widely held view that their "net community production" (Le.
their capacity to export organic carbon) is low (see Mann, 1982). According to Be et al.
(1982), the symbiotic relationship with zooxanthellae allows corals and foraminifers (see
below) to be the foremost producers of biogenic calcite in the circumglobal warm-water belt,
the former over the shallow platforms and the latter over the deep sea bed. The possible
effects of calcite precipitation by coral reefs and other organisms on atmospheric C~ is
discussed at the end of the next section. In addition, coral reefs are thought to be net exporters
of nitrate and could be indirectly fuelling the planktonic carbon pump in surrounding
oligotrophic waters.
Denitrification is the antagonistic process to nitrogen fixation, by which oxidized forms of
combined nitrogen (nitrate, nitrite and nitrous oxide) are reduced to free nitrogen gas. As in
the case of nitrogen fixation, it is generally considered that denitrification only occurs under
anaerobic or low-oxygen conditions (see Codispoti, 1989). In the open ocean, sites favourable
for denitrification would be oxygen-deficient portions of the water column, below the euphotic
layer (e.g. Ward and Zafiriou, 1988). More generally, denitrification could take place in
reducing environments (e.g. marine snow) similar to those potentially favouring N2 fixation
(see above). According to Codispoti (1989), denitrification and nitrogen fixation may be
approximately balanced on a time scale of about 107 years, but imbalances in which
denitrification may exceed nitrogen fixation could reduce export production, and thus potential
sequestration of carbon, by 20 to 30% for periods of several thousands years.
PLANKTON ORGANISMS WITH HIGH CARBON CONTENT
Estimates of carbon export from measurements of nitrogen-derived new production are
generally based upon the so-called "Redfield ratio" (e.g. Redfield, 1958; Redfield et al.,
1963), i.e. in terms of atoms C/N/P = 106116/1 (hence C/N = 6.6), which corresponds to
the average cellular composition of marine plankton 6 • However, many plankton organisms
export an extra share of carbon, either as organic thecae or in the form of calcareous skeletal
6All elemental ratios will henceforth be quoted in atoms.