XVI International Botanical Congess

We compared uptake rates determined by 15N accumulation in algal tissue and by nutrient disappearance from the medium for Ulva fenestrata and Gracilaria pacifica. With 15NH4+, atom % enrichment of thallus N increased to about 3% and about 90% of added 15N was recovered. Rates calculated by NH4+ decrease and by 15N accumulation in algal tissue differed by 20% in U. fenestrata and 32% in G. pacifica. A lag period was observed during the NO3- uptake experiments. Subsequently, 15N of thallus N increased to about 2% and recovery of 15N was complete (99.23%). Nitrate uptake rates were much lower than ammonium uptake rates. Rates calculated by NO3- decrease and by 15N accumulation in algal tissue differed by 4% in U. fenestrata and 25% in G. pacifica. Both bacterial uptake of inorganic N and release of organic N by macroalgae may contribute to the discrepancies between uptake rates calculated by 15N accumulation and disappearance of inorganic nitrogen from the medium.