PLANKTON IN NITROGEN AND PHOSPIIORUS CYCLES
103
phorus, some of which may aIso be utilized by the plants. Accordingly,
before dealing with iiutricnt assimilation by phytoplankton, wc give
here a brief account of the various chemical forms of dissolved nitrogen
and phosphorus prescnt in the sea.
A. Inorganic nitrogen
The molecular gas, N2, is the predominant form of nitrogen in the
aea. Its solubility was measured by Rakestraw and Emmel (1938) and
their data indicate that at lO"C, sea water of salinity 35%, in equilibrium with the atmosphere would contain about 500 pg-molell. The
extent to which this form of nitrogen is involved in the marine biocycle is somewhat uncertain. Benson and Parker (1961), who examined
nitrogen : argon ratios in the Atlantic Ocean, found that, within a 1%
experimental error, the measured ratios agreed with that expected
from dissolved air. They concluded that molecular nitrogen is not
utilized in the sea. Nitrogen-fixing organisms do, in fact, occur in
certain marine environments (Allen, 1963); but the supply of dissolved
carbohydrate available in the sea is probably insufficient to satisfy the
high metabolic energy requirement of continuous heterotrophic fixation
(Vaccaro, 1965). A more interesting possibility is that this energy may
be provided by sunlight in a manner similar t o photosynthesis (Fogg
and Than-Tun, 1958), and the finding that nitrogen-fixation by colonies
of the blue-green alga Trichodesmium occurred only in the light
(Dugdale et al., 1961) supports this idea.
The main inorganic forms of fixed nitrogen in the sea are the ammonium cation-the equilibrium constants for the protonation of ammonia
in solution (Sillen and Martell, 1964) indicate that at least 97% would
be present as the ion-and the nitrate and nitrite anions, nitrate being
the most highly oxidized form of nitrogen in the sea and the major
source of the element available for use by phytoplankton. The concentrations of these ions vary considerably, both seasonally and geographically (see Section XII), but in general fall within the following
ranges (Sverdrup et al., 1942): NHt-N, 0.35-3-5 pg-atoms/l; NO,-N,
0.01-3.5 pg-atomsll; and NO;-N, 0.1-43-0 pg-atomsll.
Very low Concentrations of nitrous oxide, N,O, have been detected
in the Atlantic and Pacific Oceans (Craig and Gordon, 1963), the levels
being higher in the equatorial region, possibly due to the increased
activity of denitrifying bacteria. The question of whether hydroxylamine, NH,OH, might be present in some sea waters was investigated by
Fiadeiro et al. (1967), who found that at oxygen concentrations greater
than 0.05 ml/l, it was too unstable for any appreciable concentrations
to accumulate.
103
phorus, some of which may aIso be utilized by the plants. Accordingly,
before dealing with iiutricnt assimilation by phytoplankton, wc give
here a brief account of the various chemical forms of dissolved nitrogen
and phosphorus prescnt in the sea.
A. Inorganic nitrogen
The molecular gas, N2, is the predominant form of nitrogen in the
aea. Its solubility was measured by Rakestraw and Emmel (1938) and
their data indicate that at lO"C, sea water of salinity 35%, in equilibrium with the atmosphere would contain about 500 pg-molell. The
extent to which this form of nitrogen is involved in the marine biocycle is somewhat uncertain. Benson and Parker (1961), who examined
nitrogen : argon ratios in the Atlantic Ocean, found that, within a 1%
experimental error, the measured ratios agreed with that expected
from dissolved air. They concluded that molecular nitrogen is not
utilized in the sea. Nitrogen-fixing organisms do, in fact, occur in
certain marine environments (Allen, 1963); but the supply of dissolved
carbohydrate available in the sea is probably insufficient to satisfy the
high metabolic energy requirement of continuous heterotrophic fixation
(Vaccaro, 1965). A more interesting possibility is that this energy may
be provided by sunlight in a manner similar t o photosynthesis (Fogg
and Than-Tun, 1958), and the finding that nitrogen-fixation by colonies
of the blue-green alga Trichodesmium occurred only in the light
(Dugdale et al., 1961) supports this idea.
The main inorganic forms of fixed nitrogen in the sea are the ammonium cation-the equilibrium constants for the protonation of ammonia
in solution (Sillen and Martell, 1964) indicate that at least 97% would
be present as the ion-and the nitrate and nitrite anions, nitrate being
the most highly oxidized form of nitrogen in the sea and the major
source of the element available for use by phytoplankton. The concentrations of these ions vary considerably, both seasonally and geographically (see Section XII), but in general fall within the following
ranges (Sverdrup et al., 1942): NHt-N, 0.35-3-5 pg-atoms/l; NO,-N,
0.01-3.5 pg-atomsll; and NO;-N, 0.1-43-0 pg-atomsll.
Very low Concentrations of nitrous oxide, N,O, have been detected
in the Atlantic and Pacific Oceans (Craig and Gordon, 1963), the levels
being higher in the equatorial region, possibly due to the increased
activity of denitrifying bacteria. The question of whether hydroxylamine, NH,OH, might be present in some sea waters was investigated by
Fiadeiro et al. (1967), who found that at oxygen concentrations greater
than 0.05 ml/l, it was too unstable for any appreciable concentrations
to accumulate.