METABOLIC REGULATION OF AMMONIUM UPTAKE BY ULVA RIGIDA (CHLOROPHYTA): A COMPARTMENTAL ANALYSIS OF THE RATE-LIMITING STEP FOR UPTAKE1 |
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Authors: | Rodney M Fujita Patricia A Wheeler Robert L Edwards |
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Abstract: | Non-linear time courses of ammonium (NH4+) depletion from the medium and internal accumulation of soluble nitrogen (N) in macroalgae imply that the rate-limiting step for ammonium uptake changes over time. We tested this hypothesis by measuring the time course of N accumulation in N-limited Ulva rigida C. Agardh. Total uptake was measured as removal of NH4+ from medium. Rates for the component processes (transport of NH4+ across the membrane = Rv assimilation of tissure NH4+ into soluble N compounds = Ra, assimilation of tissue NH4+ into soluble N compounds = Ra and incorporation of soluble N compounds into macromolecules = R1) were determined by measuring the rate of labelling of the major tissue N pools after the addition of 15N-ammonium. The results indicate that nitrogen-specific rates (mass N taken up / mass N present / unit time) are ranked in the order of Rt < Ra < R1 Absolute uptake rates (μmol N. mg dry wt?1. h?1) showed a different relationship. Membrane transport appears to be inhibited when NH4+ accumulates in the tissue. Maximum uptake rates occur when assimilation of NH4+ into soluble N compounds begins. Assimilation of NH4+ into soluble N compounds was initially faster than incorporation of soluble N compounds into macromolecules. Implications of rate limitations caused by differences in maximal rates and maximal pool sizes are discussed. |
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Keywords: | ammonium uptake intracellular nitrogen Ulva uptake kinetics |
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