15N measurements of ammonium and nitrate uptake by Ulva fenestrata (chlorophyta) and Gracilaria pacifica (rhodophyta):: Comparison of net nutrient disappearance, release of ammonium and nitrate, and 15N accumulation in algal tissue

Ammonium and nitrate uptake rates in the macroalgae Ulva fenestrata (Postels and Ruprecht) (Chlorophyta) and Gracilaria pacifica (Abbott) (Rhodophyta) were determined by N-15 accumulation in algal tissue and by disappearance of nutrient from the medium in long-term (4-13 days) incubations. Nitrogen-rich algae (total nitrogen > 4% dry weight [dw]) were used to detect isotope dilution by release of inorganic unlabeled N from algal thalli. Uptake of NH4+ was similar for the two macroalgae, and the highest rates were observed on the first day of incubation (45 mumol N(.)g dw(-1.)h(-1) in U. fenestrata and 32 mumol N(.)g dw(-1.)h(-1) in G. pacifica). A significant isotope dilution (from 10 to 7.9 atom % enrichment) occurred in U. fenestrata cultures during the first day, corresponding to a NH4+ release rate of I I mumol N(.)g dw(-1.)h(-1). Little isotope dilution occurred in the other algal cultures. Concurrently to net NH4+ uptake, we observed a transient free amino acid (FAA) release on the first day in both macroalgal cultures. The uptake rates estimated by NH4+ disappearance and N-15 incorporation in algal tissue compare well (82% agreement, defined as the percentage ratio of the lower to the higher rate) at high NH4+ concentrations, provided that isotope dilution is taken into account. On average, 96% of added (NH4+)-N-15 was recovered from the medium and algal tissue at the end of the incubation. Negligible uptake of NO3- was observed during the first 2-3 days in both macroalgae. The lag of uptake may have resulted from the need for either some N deprivation (use of NO3- pools) or physiological/metabolic changes required before the uptake of NO3-. During the subsequent days, NO3- uptake rates were similar for the two macroalgae but much lower than NH4+ uptake rates (1.97-3.19 mumol N(.)g dw(-1.)h(-1)). Very little isotope dilution and FAA release were observed. The agreement between rates calculated with the two different methods averaged 91% in U. fenestrata and 95% in G. pacifica. Recovery of added (NO3-)-N-15 was virtually complete (99%). These tracer incubations show that isotope dilution can be significant in NH4+ uptake experiments conducted with N-rich macroalgae and that determination of N-15 atom % enrichment of the dissolved NH4+ is recommended to avoid poor isotope recovery and underestimation of uptake rates.