Geoscience Reference
In-Depth Information
Figure 5.10
Nitrate uptake data for in
situ incubations of phytoplankton
samples at different nitrate
concentrations. The fitted curve
provided by Mike Lucas (University of
Cape Town, South Africa) and Mark
Moore (University of Southampton,
UK).
u
max
= 0.53 nmol l
-1
h
-1
0.6
0.4
0.2
k
NO
3
-
= 0.19 mmol m
-3
0
0
2
4
6
Nitrate (mmol m
-3
)
5.1.7
Nutrient uptake by phytoplankton cells
While a phytoplankton may have plenty of light for photosynthesis in the surface layer of
the ocean, limiting access to nutrients could inhibit growth. In other words, there might be
energy available to excite electrons in chlorophyll and begin the photosynthetic process,
but without nutrients the lipids and amino acids required for growth cannot be generated.
Nutrient quota in phytoplankton cells
Nutrients are absorbed from the surrounding seawater, across the cell membrane and
into the cell. Nutrients are stored within the cell, with different phytoplankton groups,
different species and even different sizes of phytoplankton within the same species
having varying capacities to store vital nutrients. One particular bacterium has been
found to store nitrate in vacuoles at concentrations 3000 to 4000 times that in ambient
seawater (McHatton et al.,
1996
). Storing nitrate in this way has been suggested as a
mechanism by which diatoms might compete for nitrate in environments with varying
nitrate supply (Stolte and Riegman,
1995
). This general ability to take up nutrients
beyond the immediate demands of photosynthesis has led to the cell quota description
of phytoplankton nutrient status and its impact on growth, first suggested by Michael
Droop on the basis of extensive laboratory experiments (Droop,
1968
). Such labora-
tory experiments are carried out on phytoplankton cultures of a single species, so that
the quota of nutrient can be expressed in terms of an amount per cell or as mass of
nutrient per mass of phytoplankton biomass.
Measuring nutrient uptake
Measurements of nutrient uptake on samples of phytoplankton collected at sea are
made using similar incubation techniques to those employed in determining
carbon fixation. Samples are 'spiked' with different quantities of filtered seawater
with a known nutrient concentration, thus increasing the samples' nutrient con-
centrations by different amounts. At the end of the incubation period the reduc-
tion in nutrients in the samples is taken as a measure of the uptake rate by the
phytoplankton. An example of an uptake curve generated by such an experiment
is shown in
Fig. 5.10
. The data have been fitted to a curve describing Michaelis-
Menten uptake kinetics:
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