Ammonium and urea uptake by some freshwater algae

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Abstract
Nitrogen deficiency increases the initial saturated rate of ammonium and urea uptake by three green and three blue-green algae. The increase caused by N deficiency is greater in darkness than in the light. Nitrogen deficiency also increases the size of the N debt in these algae. The green alga Scenedesmus quadricauda and the blue-green alga Pseudoanabaena catenata were selected for more detailed study of some factors affecting the rate of ammonium and urea uptake. The uptake of both ammonium and urea by these algae is light-stimulated; that of urea is sensitive to an uncoupler but not to the urease inhibitor acetohydroxamic acid. Ammonium and urea uptake resemble each other in being dependent on the presence of a chelator, but they differ in their pH dependence. They also differ in their dependence on substrate concentration, ammonium uptake showing half-saturation values of 0.1 to 0.3 μg atom N l−1, and urea uptake, 0.4 to 1.4 μg atom N l−1. While ammonium depresses the uptake of urea and urea depresses that of ammonium, the two substrates do not appear to compete for the same site.

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