PLANKTON IN NITROGEN AND PHOSPHORUS CYCLES
119
S. N H t concentration (wM)
FIO. 3. The rate of ammonium uptake (v) by Rhirosolenin robilala as a function of the
ammonium concentration (S) in the medium, and the linear t.ranaformation of the
data which provides the haltsaturation constant. (After Eppley el. al., 1969b.)
The form of this equation is the same as that occurring in the
Langmuir adsorption isotherm and in Michaelis-Menten enzyme
kinetics; but as the mechanism of uptake is unknown there is no
justification at the present time for assuming that the uptake equation
results directly from surface or enzyme promoted nutrient transport,
though both will undoubtedly be involved.
Values of the two constants may be obtained using a linear transformation of the equation, for which several forms are possible, that
used by Eppley et al. being
8 = NV,,,(S/v) - K,".
Thus, for a constant population N, plots of Slv against S should be linear,
the intercept on the abscissa corresponding to -K:. The results of such
a transformation are also illustrated in Figs. 2 and 3, and the values of
K: obtained by Eppley et al. in this way are reproduced in Table IV,
together with those obtained for natural populations by MacIsaac and
Dugdale (1969). Eppley et al. (196913) have pointed out several interesting features in the data : (1) that a high K," value for nitrate uptake is
usually associated with a high value for ammonia uptake; (2) that
larger species, which tend to grow more slowly, in general have higher
119
S. N H t concentration (wM)
FIO. 3. The rate of ammonium uptake (v) by Rhirosolenin robilala as a function of the
ammonium concentration (S) in the medium, and the linear t.ranaformation of the
data which provides the haltsaturation constant. (After Eppley el. al., 1969b.)
The form of this equation is the same as that occurring in the
Langmuir adsorption isotherm and in Michaelis-Menten enzyme
kinetics; but as the mechanism of uptake is unknown there is no
justification at the present time for assuming that the uptake equation
results directly from surface or enzyme promoted nutrient transport,
though both will undoubtedly be involved.
Values of the two constants may be obtained using a linear transformation of the equation, for which several forms are possible, that
used by Eppley et al. being
8 = NV,,,(S/v) - K,".
Thus, for a constant population N, plots of Slv against S should be linear,
the intercept on the abscissa corresponding to -K:. The results of such
a transformation are also illustrated in Figs. 2 and 3, and the values of
K: obtained by Eppley et al. in this way are reproduced in Table IV,
together with those obtained for natural populations by MacIsaac and
Dugdale (1969). Eppley et al. (196913) have pointed out several interesting features in the data : (1) that a high K," value for nitrate uptake is
usually associated with a high value for ammonia uptake; (2) that
larger species, which tend to grow more slowly, in general have higher