Uptake kinetics and assimilation of inorganic nitrogen by Catenella nipae and Ulva lactuca

Abstract

The kinetics of NH 4 +, the assimilation of NH 4 + and nitrate uptake by Catenella nipae (Rhodophyta) were compared with Ulva lactuca (Chlorophyta). Both algal species demonstrated saturable NH 4 + and nitrate uptake kinetics. Uptake of NH 3 by simple diffusion across the plasmalemma could not account for the observed saturation uptake kinetics of ammonia-N (NH 3+NH 4 +), so NH 4 + was the chemical form being taken up by the transport systems of the cells. Although the V max of NH 4 + uptake by C. nipae and U. lactuca was high (≈550 and 450 μmol g −1 DW h −1, respectively), the K m for U. lactuca (≈85 μM) was much lower than that for C. nipae (≈692 μM). The K m and V max values for nitrate uptake were much lower than for NH 4 + for both C. nipae ( K m≈5 μM; V max≈8.3 μmol g −1 DW h −1) and U. lactuca ( K m≈34 μM; V max≈116 μmol g −1 DW h −1). Over the incubation times used (up to 28 min) there was no apparent induction of nitrate transport in either species. There was no evidence for induction of NH 4 + transport in C. nipae but incubation time did affect the kinetics of NH 4 + uptake in U. lactuca. At high concentrations of NH 4 +, U. lactuca rapidly assimilated it into organic N with limited build-up of intracellular NH 4 + whereas C. nipae accumulated large amounts of NH 4 + because uptake of NH 4 + overtook the rate of assimilation. The effects of species-specific differences and experimental design on uptake-kinetic estimates are discussed in the light of the results of this other comparable studies. C. nipae is promising as a bioindicator species of the N-status of estuaries but U. lactuca changes its N-status too quickly for it to be a useful bioindicator of environmental conditions.