title: Uptake of [14C]methylammonium by plankton communities: a comparative assay for ammonium transport systems in natural waters.
abstract: A diverse range of freshwater plankton communities were tested for their ability to take up [14C]methylammonium. Uptake occurred at low substrate levels by high-affinity, energy-requiring, transport systems which were competitively inhibited by ammonium but not by L-amino acids or nicotinamide. A simple competitive inhibition model was used to examine the effects of increasing ammonium levels on uptake in a eutrophic lake. Apparent K1 values for the labelled substrate markedly increased with increasing ammonium. The transport systems had an approximately five-fold greater affinity for ammonium than for methylammonium. The Vmax for methylammonium uptake was relatively insensitive to large changes in ambient ammonium levels. This kinetic parameter may be a useful comparative measure of ammonium transport capacity in natural waters, particularly where low ambient ammonium concentrations preclude the use of 15N.
mesh_terms: D000596:Amino Acids / Q000494:pharmacology; D000643:Ammonium Chloride / Q000494:pharmacology; D001693:Biological Transport, Active / Q000187:drug effects; D003624:Darkness; D007700:Kinetics; D008027:Light; D010933:Plankton / Q000378:metabolism*; D000644:Quaternary Ammonium Compounds / Q000378:metabolism*; D014871:Water Microbiology
chemical_list: D000596:Amino Acids; D000644:Quaternary Ammonium Compounds; D000643:Ammonium Chloride