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

The kinetics of NH4+, the assimilation of NH4+ and nitrate uptake by Catenella nipae (Rhodo-phyta) were compared with Ulva lactuca (Chlorophyta). Both algal species demonstrated saturable NH4+ and nitrate uptake kinetics. Uptake of NH3 by simple diffusion across the plasmalemma could not account for the observed saturation uptake kinetics of ammonia-N (NH3 + NH4+), so NH4+ was the chemical form being taken up by the transport systems of the cells. Although the V, a, of NH4+. uptake by C. nipae and U. lactuca was high (approximate to550 and 450 mumol g(-1) DW h(-1), respectively), the K-m for U. lactuca (approximate to85 muM) was much lower than that for C. nipae (approximate to692 muM). The K-m and V-max values for nitrate uptake were much lower than for NH4+ for both C. nipae (K-m approximate to 5 muM; V-max 8.3 mumol g(-1) DW h(-1)) and U. lactuca (K-m approximate to 34 muM; V-max approximate to 116 mumol 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 NH4+ transport in C. nipae but incubation time did affect the kinetics of NH4+ uptake in U. lactuca. At high concentrations of NH4+, U. lactuca rapidly assimilated it into organic N with limited build-up of intracellular NH4+ whereas C. nipae accumulated large amounts of NH4+ because uptake of NH4+ 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. (C) 2003 Elsevier Science B.V. All rights reserved.