Phytoplankton are generally capable of active uptake of DIN from external concentrations as low as 3-4 mg N m-3 (0.2-0.3 |M). Although nitrate is usually the most abundant of the DIN sources in the surface waters of lakes and seas, ammonium is taken up preferentially if concentrations exceed some 0.15-0.5 |M N (2-7 mg N m-3). This is because the initial intracellu-lar of assimilation of nitrogen proceeds via a reductive amination, forming glutamate, and a subsequent transamination to form other amino acids. The substrate is apparently always ammonium (Owens and Esaias, 1976). Thus, it is both probable and energetically preferable that the alga should use ammonium directly; nitrate and nitrite have to be reduced prior to assimilation in reactions catalysed by (respectively) nitrate reduc-tase and nitrite reductase, so adding to the energetic cost of nitrogen metabolism. This difference in the energy requirement for the assimilation of nitrate and ammonium is reflected in the photosynthetic quotient, being about 1.1 mol O2 (mol CO2)-1 when ammonium is assimilated and 1.4 when nitrate is the substrate (Geider and McIntyre, 2002, quoting Laws, 1991). Eppley et al. (1969a) devised an assay for nitrate-reductase activity in natural populations and which shows, consistently, that it is suppressed by ammonia concentrations exceeding 0.5-1.0 | g-atoms N L-1 (0.5-1.0 |M N) (see also McCarthy et al., 1975). More recently, the genes encoding the kinases for bacterial nitrogen transport have been recognised (Stock et al., 1989) and the action of ammonium in suppressing them has been similarly demonstrated (Vega-Palas et al., 1992).
The kinetics of DIN uptake by marine phy-toplankton have been studied extensively; those of freshwater species having received relatively less attention. Half-saturation concentrations for uptake (Ku) by named, small-celled oceanic species in culture (Eppley et al., 1969b; Caperon and Meyer, 1972; Parsons and Takahashi, 1973) fall within the range 0.1-0.7 |M N (when nitrate is the substrate) and 0.1-0.5 |M N (with ammonium). Among neritic diatoms, the corresponding ranges are 0.4-5.1 |M NO3.N and 0.5-9.3 |M NH4.N. Some half-saturation concentrations for nitrate uptake among freshwater plankters are available (Lehman et al., 1975; Reynolds, 1987a; Sommer, 1994), typically falling in the range 0.3-3.0 |M N. The maximum rates of DIN uptake at 20 °C (calculated to be generally equivalent to 0.6 to 35 |imol N (mol cell C)-1 s- ^ are competent to saturate growth demand (D/S < 0.1 to 0.2: Riebesell and Wolf-Gladrow, 2002). As in the general case (see Section 4.2.3), uptake and consumption achieve parity at steady rates of growth, at external DIN concentrations generally <7 |imol N L-1.
Conversely, nitrogen availability is unlikely to constrain phytoplankton activity and growth before the DIN concentration in the medium falls to below 7 |imol N L-1 (~100 mg N m-3) in the case of large, low-affinity species or below ~0.7 |mol N L-1 (~10 mg N m-3) in the case of oceanic picoplankton). Activities become severely constrained once the cell nitrogen content falls below ~0.07 mol N (mol C)-1, when the cell reacts to its internal N deficiency by closing down non-essential processes. The minimum cell quota (q0) of nitrogen in phytoplankton cells is said to be 0.02-0.05 mol N (mol C)-1 (Sommer, 1994).
Applying the statistic (K/q0), the ultimate yield or carrying capacity of the available inorganic combined nitrogen is around 20 mol C (mol N)-1, with a possible extreme of ~50 (1742 g C : g N). Before the internal nitrogen becomes yield limiting, however, the equivalence is not likely to much exceed 10 and perhaps as little as 5 mol C (mol N)-1 (say, 8.5 to 4.2 g C : g N). In terms of chlorophyll yield, the supportive capacity of 5-20 mol C (mol N)-1 is equivalent to some 0.08-0.34 g chla : g N. The factor used in the capacity-solving model of Reynolds and Maberly (2002), which is biassed by data from systems that are more likely to be P-deficient, is 0.11 g chla: g N.
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