Estimation of kinetic parameters for the transport of nitrate and ammonium into marine phytoplankton

Abstract

Methods for the determination of kinetic parameters for the transport of ammonium and nitrate into phytoplankton are considered using simulations generated by the ammonium-nitrate interaction model (ANIM; Flynn et al. 1997, Phil Trans R Soc Lond B 352:1625-1645). Problems associated with experimental conditions such as incubation periods, non-constant substrate concentrations (including variations in the isotope ratio in 15N assays), presence of the counter nutrient, and different cellular N status are considered. Depending on the period of incubation and the biomass, errors occur which can result in an over or underestimate of the half saturation constant for transport (K(t)), while the maximum transport rate (V(t)(max)) 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 studies 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 estimating the initial (zero time) transport rate. Simulations within a simple 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(t)(max)/K(t) for success of the algae. Estimates of transport kinetics for different species, and other factors such as cell size and shape which may affect transport, should not be used alone to infer a competitive advantage or disadvantage for the individual species. These results have similar implications for studies of nutrient transport into other microbes such as bacteria and mixotrophic protists.