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METABOLIC REGULATION OF AMMONIUM UPTAKE BY ULVA RIGIDA (CHLOROPHYTA): A COMPARTMENTAL ANALYSIS OF THE RATE‐LIMITING STEP FOR UPTAKE 1
Author(s) -
Fujita Rodney M.,
Wheeler Patricia A.,
Edwards Robert L.
Publication year - 1988
Publication title -
journal of phycology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.85
H-Index - 127
eISSN - 1529-8817
pISSN - 0022-3646
DOI - 10.1111/j.1529-8817.1988.tb04262.x
Subject(s) - chlorophyta , biology , limiting , ammonium , botany , algae , mechanical engineering , chemistry , organic chemistry , engineering
Non‐linear time courses of ammonium (NH 4 + ) 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 NH 4 + from medium. Rates for the component processes (transport of NH 4 + across the membrane = R v assimilation of tissure NH 4 + into soluble N compounds = R a , assimilation of tissue NH 4 + into soluble N compounds = R a and incorporation of soluble N compounds into macromolecules = R 1 ) were determined by measuring the rate of labelling of the major tissue N pools after the addition of 15 N‐ammonium. The results indicate that nitrogen‐specific rates (mass N taken up / mass N present / unit time) are ranked in the order of R t < R a < R 1 Absolute uptake rates (μmol N. mg dry wt −1 . h −1 ) showed a different relationship. Membrane transport appears to be inhibited when NH 4 + accumulates in the tissue. Maximum uptake rates occur when assimilation of NH 4 + into soluble N compounds begins. Assimilation of NH 4 + 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.