ESTIMATION OF KINETIC-PARAMETERS FOR THE TRANSPORT OF NITRATE AND AMMONIUM INTO MARINE-PHYTOPLANKTON

Methods for the determination of kinetic parameters for the transport of ammonium and nitrate into phytoplankton are considered using simula tions generated by the ammonium-nitrate interaction model (ANIM; Flynn et al. 1997, Phil Trans R Soc Lond B 352:1625-1645). Problems associa ted with experimental conditions such as incubation periods, non-const ant substrate concentrations (including variations in the isotope rati o in N-15 assays), presence of the counter nutrient, and different cel lular N status are considered. Depending on the period of incubation a nd the biomass, errors occur which can result in an over or underestim ate of the half saturation constant for transport (K-t), while the max imum transport rate (V-tmax) is underestimated unless derepression of nutrient transport and/or assimilation occurs. Because of the effects of intracellular feedback processes on transport, there may be little advantage in conducting stud;es using stable substrate concentrations rather than determining transport rates from the decrease in substrate concentrations. Differentiating quadratic curve fits through plots of cumulative transport against time provides a simple method for estima ting the initial (zero time) transport rate. Simulations within a simp le photic zone model run using different kinetic parameters for algal nutrient transport indicate that post-transport processes are, within reasonable bounds, likely to be at least as important as V-tmax/K-t fo r success of the algae. Estimates of transport kinetics for different species, and other factors such as cell size and shape which may affec t transport, should not be used alone to infer a competitive advantage or disadvantage for the individual species. These results have simila r implications for studies of nutrient transport into other microbes s uch as bacteria and mixotrophic protists.