Rapid ammonium uptake and nitrogen interactions in five intertidal seaweeds grown under field conditions

Abstract

Time courses of ammonium and nitrate uptake rates were determined for five intertidal macrophytes collected directly from the field. There were two perennials, Pelvetiopsis limitata Gardner and Fucus gardneri Silva, and three annuals, Mastocarpus papillatus (C. Ag. Kutzing, Gracilaria pacifica Abbott and Enteromorpha intestinalis (L.) Grev. When a 15-μM pulse of ammonium was added to the incubation medium, all three annuals and one of the two perennials, Fucus gardneri, showed enhanced ammonium uptake rates for the first 15 min of a 30-min exposure. This is the first report of non-linear nutrient depletion by plants grown in the field. There was no enhancement in the initial nitrate uptake rate. In fact, nitrate uptake had to be induced in the low intertidal Gracilaria pacifica by incubating it in 30 μM nitrate for 20 min. All species took up both ammonium and nitrate simultaneously. Enteromorpha intestinalis and the high intertidal Gracilaria pacifica exhibited a 50% decrease in nitrate uptake in the presence of ammonium. Ammonium concentrations of > 5 μM were required to inhibit nitrate uptake in the former.