á P . M . Glibert Interactions between NH + 4 and NO ) 3 uptake and assimilation : comparison of diatoms and dino ̄ agellates at several growth temperatures

Ammonium concentrations of 1 lM are commonly cited as being the threshold for inhibition of NO3 uptake, but the applicability of this threshold to phytoplankton from di€erent taxonomic classes has rarely been examined. Additionally, little is known about the in ̄uence of environmental variables (e.g. growth temperature) on the interaction between ambient NH4 and NO ÿ 3 uptake. Four species of estuarine phytoplankton, two diatom [Chaetoceros sp., and Thalassiosira weiss ̄ogii (Grunow) Fryxell et Hasle] and two dino ̄agellate [Prorocentrum minimum (Pavillard) Schiller, and Gyrodinium uncatenum Hulburt], were grown on NO3 at several di€erent temperatures (4, 10, 15, or 20 °C), and the impact of NH4 additions on NO ÿ 3 uptake/assimilation (non-TCA-extracted) and assimilation (TCA-extracted) was assessed. For all species at all temperatures, NO3 uptake/assimilation and assimilation rates decreased in a roughly exponential manner with increasing NH4 concentrations but were not completely inhibited even at elevated NH4 concentrations of 200 lM. Estimated half-inhibition concentrations (Ki) were signi®cantly greater in the diatom species (mean ‹ SE; 2.70 ‹ 0.67 lM) than in the dino ̄agellate species (1.26 ‹ 0.55 lM). Half-inhibition constants were positively related to temperature-limited relative growth rate although not signi®cantly. The observed inhibition of NO3 uptake and assimilation, as a percentage of NO3 uptake in the absence of NH ‡ 4 , averaged about 80% and ranged from 49 to 100%. For all species, a signi®cant (P < 0.001) positive correlation was found between percent inhibition of NO3 assimilation and temperature-limited relative growth rate. Two experiments on Chesapeake Bay phytoplankton during an April 1998 diatom bloom showed that in short-term ( 1 h) temperature manipulation experiments, percent inhibition of NO3 uptake/assimilation was also positively related (P = 0.05) to experimental temperature. The observed relationships between temperature-limited relative growth rate and percent inhibition of NO3 assimilation rates for the species tested suggest that at the enzyme level, the inhibitory mechanism of NO3 assimilation is similar among species, but at the whole cell level may be regulated by species-speci®c di€erences in the accumulation of internal metabolites. These ®ndings add not only to our understanding of species-speci®c variability and the role of growth temperature, but also provide additional data with which to evaluate current models of NH4 and NO ÿ 3 interactions.