P W K T O N IN NITROGEN AND PHOSPHORUS CYCLES
117
the light, though at a lower rate. Thus, for tropical phytoplankton the
specific rate of ammonia uptake in the dark was only 0.6 of the light
value, and for nitrate the fraction was only 0.3. There is in fact evidence
that light is involved in nitrate reduction within plant cells. Thus,
Eppley and Coatsworth (1968) found that cultures of Ditylum brightwellii containing little nitrate reductase took up nitrate in both the
dark and the light. However, in the dark, the rate of uptake decreased
over a period of two hours and most (84%) of the nitrate accumulated
could be recovered from the cells unchanged. In the light, on the other
hand, the rate of uptake remained constant over the same period and
only 44% could be recovered. Further, as no nitrite could be detected
in the cells, Eppley and Coatsworth concluded that the reduction had
proceeded beyond this stage and pointed out t h a t the rate of reduction
(0.5 pg-mole/lOs cells/h) was higher than could be accounted for by the
low level of nitrate reductase activity within the cells. It therefore
appeared that a light-induced reduction was occurring. This view was
supported by the previous work of Grant (1967) who showed that the
rate of nitrate assimilation by Dunuliella tertiolecta Butch. is a function
of light energy.
The effect of light upon nitrite uptake is less clear, for whereas
Ditylum brightwellii did not assimilate this form of nitrogen in the dark
(Eppley and Coatsworth, 1968), Dunuliella tertiolecta did (Grant, 1967),
and Vaccaro and Ryther (1960) found that at low light energies,
phytoplankton may even secrete nitrite.
C . The hyperbolic relationship
Probably tho first investigation of the rate of nutrient uptake by a
marine phytoplankton species was made by Ketchum (1939a). Using
cultures of the diatom " Nitzschia closterium forma minutissimu " (now
P k m t y l u m tricornutum Bohlin) it was shown that the rate of nitrate
uptake in the light, measured over a short period, varied hyperbolically
with the nitrate concentration in the medium, increasing rapidly with
rising concentration at low nitrate levels, then less quickly at higher
levels and finally reaching a maximum value above about 14 pg-atoms
Until recently, evidence that this hyperbolic relationship between
the rate of uptake of a nutrient and its concentration holds generally
waa still limited ; but for nitrogen compounds it has now been shown to
apply to nitrite and nitrate assimilation by Ditylum brightwellii (Eppley
and Coatsworth, 1968), and for the uptake of nitrate and ammonia by
a wide range of species in culture (Table IV) (Eppley et al., 196913).
In addition, MacIsaac and Dugdale (1969) were able to demonstrate,
NOi-N/l.
117
the light, though at a lower rate. Thus, for tropical phytoplankton the
specific rate of ammonia uptake in the dark was only 0.6 of the light
value, and for nitrate the fraction was only 0.3. There is in fact evidence
that light is involved in nitrate reduction within plant cells. Thus,
Eppley and Coatsworth (1968) found that cultures of Ditylum brightwellii containing little nitrate reductase took up nitrate in both the
dark and the light. However, in the dark, the rate of uptake decreased
over a period of two hours and most (84%) of the nitrate accumulated
could be recovered from the cells unchanged. In the light, on the other
hand, the rate of uptake remained constant over the same period and
only 44% could be recovered. Further, as no nitrite could be detected
in the cells, Eppley and Coatsworth concluded that the reduction had
proceeded beyond this stage and pointed out t h a t the rate of reduction
(0.5 pg-mole/lOs cells/h) was higher than could be accounted for by the
low level of nitrate reductase activity within the cells. It therefore
appeared that a light-induced reduction was occurring. This view was
supported by the previous work of Grant (1967) who showed that the
rate of nitrate assimilation by Dunuliella tertiolecta Butch. is a function
of light energy.
The effect of light upon nitrite uptake is less clear, for whereas
Ditylum brightwellii did not assimilate this form of nitrogen in the dark
(Eppley and Coatsworth, 1968), Dunuliella tertiolecta did (Grant, 1967),
and Vaccaro and Ryther (1960) found that at low light energies,
phytoplankton may even secrete nitrite.
C . The hyperbolic relationship
Probably tho first investigation of the rate of nutrient uptake by a
marine phytoplankton species was made by Ketchum (1939a). Using
cultures of the diatom " Nitzschia closterium forma minutissimu " (now
P k m t y l u m tricornutum Bohlin) it was shown that the rate of nitrate
uptake in the light, measured over a short period, varied hyperbolically
with the nitrate concentration in the medium, increasing rapidly with
rising concentration at low nitrate levels, then less quickly at higher
levels and finally reaching a maximum value above about 14 pg-atoms
Until recently, evidence that this hyperbolic relationship between
the rate of uptake of a nutrient and its concentration holds generally
waa still limited ; but for nitrogen compounds it has now been shown to
apply to nitrite and nitrate assimilation by Ditylum brightwellii (Eppley
and Coatsworth, 1968), and for the uptake of nitrate and ammonia by
a wide range of species in culture (Table IV) (Eppley et al., 196913).
In addition, MacIsaac and Dugdale (1969) were able to demonstrate,
NOi-N/l.