TRANSIENT AMMONIUM UPTAKE IN THE MACROALGA ULVA-LACTUCA (CHLOROPHYTA)- NATURE, REGULATION, AND THE CONSEQUENCES FOR CHOICE OF MEASURING TECHNIQUE

The nature of transient ammonium uptake by the macroalga Ulva lactuca L. was studied from the depletion of ammonium after single additions o f ammonium to batch cultures. The experiments were carried out by the application of two different experimental setups: the ''multiple flask '' and the ''perturbation'' techniques. Uptake rate was nonlinear with time, and three distinct, succeeding phases of uptake were identified : 1) ''surge'' uptake, i.e. transiently enhanced uptake that lasted fo r a few hours only,followed by 2) ''internally'' controlled uptake a r elatively constant phase occurring at high substrate concentrations, a nd finally 3) the ''externally'' controlled uptake phase, which was su bstrate-dependent and occurred at low substrate concentrations. Surge uptake occurred over a broad range of substrate concentrations but was concentration-dependent and, so, equalled externally controlled uptak e rates at substrate concentrations below 3-10 mu M. The transient nat ure of ammonium uptake rate seemed related to rapid changes in small i ntracellular pools of inorganic nitrogen or amino acids rather than to changes in total N content of the algae. The transient nature of ammo nium uptake has important implications for the measurement of uptake r ates when either of the two standard methods, the multiple flask and t he perturbation technique, are used, and I recommend that a combinatio n of the two methods be used for future uptake experiments.