PLANKTON IN NITROGEN AND PHOSPHORUS CYCLES
115
not likely to be of much significance in the marine nitrogen cycle because
of their very low levels in the sea. Accordingly, our attention will be
directed towards the inorganic forms of nitrogen only.
When nitrate is utilized as a nutrient, it is reduced stepwise within
the plant cells through nitrite, hyponitrite, hydroxylamine and ammonia
before the nitrogen is finally incorporated into the cellular material
(Nicholas, 1959). This reduction is an energy-consuming process, the
overall reaction NO, + H + + 2H20---tNHf + OH- + 2 0 2 involving a free energy increase of 77.4 k cals/mole at 25OC (Cooper, 19373).
It is not surprising, therefore, t o find that when both ammonia and
nitrate are present in culture media, ammonia is used preferentially by
phytoplankton. The most recent demonstrations of this are those of
Grant et al. (1967) using the diatom Cylindrothecu closterium var.
Culifornica (Mereschk.) Reimann et Lewin, and of Strickland et al.
(1969) who worked with large-scale, deep-tank cultures of Ditylum
brightwellii (T. West) van Herck, Cachoninu niei A. R. Loeblich, 111,
and a mixture of Gonyaulux polyedra Stein and Phaeocystis sp.
The variation in the concentrations of the two forms of nitrogen
observed by Strickland et al. during the growth of the dinoflagellate
Cachonina niei is illustrated in Fig. 1. The increase in chlorophyll a
resulting from plant growth was accompanied in the early stages by a
decrease in the ammonia concentration only; but on the sixth day,
when the ainnionium-nitrogen had been reduced t o about 1 pg-atom/l.,
nitrate and ammonia were both utilized. Similar events occurred in
the other deep-tank cultures.
Eppley et al. (1969e) using the same cultures as Strickland et al.
found that nitrate assimilation was associated with the production,
within the cells, of the enzyme nitrate reductase (Fig. 1). This enzyme
was initially absent, as the stock cultures were grown on nitrite. On
the eighth day of the Cachoninu culture, the cells were allowed to
migrate to the surface by stopping the mixing and it was possible to
replace most of the original culture medium by sea water of low nitrate
content without losing the phytoplankton. This resulted in the almost
complete disappearance of the nitrate reductase from the cells until
addition of nitrate to the culture on the 13th day caused a dramatic
rise in the enzyme activity.
Eppley et al. have suggested that the discrimination between nitrate
and amnionia as nitrogen sources was because the formation of nitrate
reductase was repressed a t the levels of ammonia initially present in
the culture medium. I n the sea, on the other hand, ammonia concentrations are normally so low that this repression should not occur, and
Eppley et al. were able t o detect nitrate reductase activity in sea water
A.Y.B.--B
6
115
not likely to be of much significance in the marine nitrogen cycle because
of their very low levels in the sea. Accordingly, our attention will be
directed towards the inorganic forms of nitrogen only.
When nitrate is utilized as a nutrient, it is reduced stepwise within
the plant cells through nitrite, hyponitrite, hydroxylamine and ammonia
before the nitrogen is finally incorporated into the cellular material
(Nicholas, 1959). This reduction is an energy-consuming process, the
overall reaction NO, + H + + 2H20---tNHf + OH- + 2 0 2 involving a free energy increase of 77.4 k cals/mole at 25OC (Cooper, 19373).
It is not surprising, therefore, t o find that when both ammonia and
nitrate are present in culture media, ammonia is used preferentially by
phytoplankton. The most recent demonstrations of this are those of
Grant et al. (1967) using the diatom Cylindrothecu closterium var.
Culifornica (Mereschk.) Reimann et Lewin, and of Strickland et al.
(1969) who worked with large-scale, deep-tank cultures of Ditylum
brightwellii (T. West) van Herck, Cachoninu niei A. R. Loeblich, 111,
and a mixture of Gonyaulux polyedra Stein and Phaeocystis sp.
The variation in the concentrations of the two forms of nitrogen
observed by Strickland et al. during the growth of the dinoflagellate
Cachonina niei is illustrated in Fig. 1. The increase in chlorophyll a
resulting from plant growth was accompanied in the early stages by a
decrease in the ammonia concentration only; but on the sixth day,
when the ainnionium-nitrogen had been reduced t o about 1 pg-atom/l.,
nitrate and ammonia were both utilized. Similar events occurred in
the other deep-tank cultures.
Eppley et al. (1969e) using the same cultures as Strickland et al.
found that nitrate assimilation was associated with the production,
within the cells, of the enzyme nitrate reductase (Fig. 1). This enzyme
was initially absent, as the stock cultures were grown on nitrite. On
the eighth day of the Cachoninu culture, the cells were allowed to
migrate to the surface by stopping the mixing and it was possible to
replace most of the original culture medium by sea water of low nitrate
content without losing the phytoplankton. This resulted in the almost
complete disappearance of the nitrate reductase from the cells until
addition of nitrate to the culture on the 13th day caused a dramatic
rise in the enzyme activity.
Eppley et al. have suggested that the discrimination between nitrate
and amnionia as nitrogen sources was because the formation of nitrate
reductase was repressed a t the levels of ammonia initially present in
the culture medium. I n the sea, on the other hand, ammonia concentrations are normally so low that this repression should not occur, and
Eppley et al. were able t o detect nitrate reductase activity in sea water
A.Y.B.--B
6