CONTENTS page 1. Introduction 1625 2. Theoretical bases and model development 1626 (a) Model construction 1626 (b) Control of transport (figures 2, 3; equation 5.x in table 5) 1629 (c) Nitrate reduction to ammonium (figures 2, 3; equation 6.x in table 6) 1630 (d) Assimilation of intracellular ammonium and growth (figures 2, 3; equation 7.x in table 7) 1633 (e) Operation in light-dark cycles 1635 (f) Parametrization 1635 3. Results 1635 (a) Simulation of ammonium-nitrate interactions 1635 (b) Sensitivity analysis 1637 4. Discussion 1639 References 1643 SUMMARY An empirically based mathematical model is presented which can simulate the major features of the interactions between ammonium and nitrate transport and assimilation in phytoplankton. The model (ammonium-nitrate interaction model), which is configured to simulate a generic microalga rather than a specified species, is constructed on simplified biochemical bases. A major requirement for parametrization is that the N:C ratio of the algae must be known and that transport and internal pool sizes need to be expressed per unit of cell C. The model uses the size of an internal pool of an early organic product of N assimilation (glutamine) to regulate rapid responses in ammoniumnitrate interactions. The synthesis of enzymes for the reduction of nitrate through to ammonium is induced by the size of the internal nitrate pool and repressed by the size of the glutamine pool. The assimilation of intracellular ammonium (into glutamine) is considered to be a constitutive process subjected to regulation by the size of the glutamine pool. Longer term responses have been linked to the nutrient history of the cell using the N:C cell quota. N assimilation in darkness is made a function of the amount of surplus C present and thus only occurs at low values of N:C. The model can simulate both qualitative and quantitative temporal shifts in the ammonium-nitrate interaction, while inclusion of a derivation of the standard quota model enables a concurrent simulation of cell growth and changes in nutrient status.
No abstract
Summary Leaf tissue from over 100 species, representing 40 families (mostly native British plants from natural populations), was assayed for in vivo nitrate reductase (NR) activity. From studies on the optimal assay conditions for the leaf tissue of 18 species, an assay procedure suitable for most species was devised. Higher levels of NR activity were measured in the leaves of ruderal species (mean NR activity = 4.39 ± 0.84 μmolh−1 g−1 fresh weight) than in woodland‐edge species (mean NR activity = 1.36 ± 0.29 μmolh−1 g−1 fresh weight). A survey of NR activity in the leaves of 41 woody species showed that most contained significant levels of NR activity (> 1.0μmol h−1 g−1 fresh weight) suggesting that nitrate assimilation was appreciable in these species.
Nitrification is a key stage in the nitrogen cycle; it enables the transformation of nitrogen into an oxidized, inorganic state. The availability of nitrates produced by this process often limits primary productivity and is an important determinant in plant community ecology and biodiversity. Chemoautotrophic prokaryotes are recognized as the main facilitators of this process, although heterotrophic nitrification by fungi may be significant under certain conditions. However, there has been neither biochemical nor ecological evidence to support nitrification by photoautotrophic plants. Here we show how certain legumes that accumulate the toxin, 3-nitropropionic acid, generate oxidized inorganic nitrogen in their shoots, which is returned to the soil in their litter. In nitrogen-fixing populations this 'new' nitrate and nitrite can be derived from the assimilation of nitrogen gas. Normally, the transformation of elemental nitrogen from the atmosphere into a fixed oxidized form (as nitrate) is represented in the nitrogen cycle as a multiphasic process involving several different organisms. We show how this can occur in a single photoautotrophic organism, representing a previously undescribed feature of this biogeochemical cycle.
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