MEPS 249:133-144 (2003)  -  doi:10.3354/meps249133

ABSTRACT: Nitrogen metabolism in Enteromorpha intestinalis from 3 contrasting populations in the Auckland region, New Zealand, was investigated. The sites (intertidal flats at Laingholm in the Manukau Harbour and Mangemangeroa and rockpools at Waterfall Reef) were chosen as they provided a range of ammonium enrichment. Seawater at Laingholm had higher levels of ammonium and total inorganic nitrogen compared with the other 2 sites. However, mean nitrate levels in Mangemangeroa seawater were similar to those measured at Laingholm. Seawater at Waterfall Reef had the lowest concentrations of ammonium and nitrate. At all 3 sites, E. intestinalis was found high in the intertidal. At Laingholm, the ammonium-enriched site, plants were immersed for 90 min at high tide; during this period the plants took up sufficient ammonium to increase their nitrogen content by 10%. The higher levels of inorganic nitrogen in the seawater at Laingholm resulted in higher levels of tissue nitrogen, chl a and b, glutamine and asparagine, and lower nitrogen-specific rates of ammonium assimilation (measured in the laboratory in the presence of saturating concentrations of ammonium) in E. intestinalis at this site. There were strong positive correlations between seawater concentrations of ammonium and the level of glutamine and between levels of chl a and b and tissue nitrogen in E. intestinalis. The nitrogen-specific rate of ammonium assimilation decreased as the nitrogen content of the plant increased and reached a minimum value above a nitrogen content of 1 to 2% or a glutamine level of 5 µmol per g dry weight (DW). This glutamine level corresponded to a seawater ammonium concentration of 6 µM. The nitrogen-specific rate of ammonium assimilation provides a novel and important bioindicator of the link between the concentration of inorganic nitrogen in the environment and the nitrogen status of the plant.

KEY WORDS: Amino acids · Ammonium assimilation · Bioindicators · Chlorophyll · Enteromorpha intestinalis · Nitrogen status