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15 N 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 15 N ACCUMULATION IN ALGAL TISSUE 1
Author(s) -
Naldi Mariachiara,
Wheeler Patricia A.
Publication year - 2002
Publication title -
journal of phycology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.85
H-Index - 127
eISSN - 1529-8817
pISSN - 0022-3646
DOI - 10.1046/j.1529-8817.2002.01070.x
Subject(s) - chlorophyta , thallus , algae , biology , ammonium , nitrate , nutrient , incubation , nitrogen , botany , isotope dilution , dilution , dry weight , zoology , environmental chemistry , ecology , chemistry , biochemistry , chromatography , mass spectrometry , physics , organic chemistry , thermodynamics
Ammonium and nitrate uptake rates in the macroalgae Ulva fenestrata (Postels and Ruprecht) (Chlorophyta) and Gracilaria pacifica (Abbott) (Rhodophyta) were determined by 15 N 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 NH 4 + was similar for the two macroalgae, and the highest rates were observed on the first day of incubation (45 μmol N·g dw − 1 ·h − 1 in U. fenestrata and 32 μmol 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 NH 4 + release rate of 11 μmol N·g dw − 1 ·h − 1 . Little isotope dilution occurred in the other algal cultures. Concurrently to net NH 4 + uptake, we observed a transient free amino acid (FAA) release on the first day in both macroalgal cultures. The uptake rates estimated by NH 4 + disappearance and 15 N incorporation in algal tissue compare well (82% agreement, defined as the percentage ratio of the lower to the higher rate) at high NH 4 + concentrations, provided that isotope dilution is taken into account. On average, 96% of added 15 NH 4 + was recovered from the medium and algal tissue at the end of the incubation. Negligible uptake of NO 3 − 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 NO 3 − pools) or physiological/metabolic changes required before the uptake of NO 3 − . During the subsequent days, NO 3 − uptake rates were similar for the two macroalgae but much lower than NH 4 + uptake rates (1.97–3.19 μmol 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 15 NO 3 − was virtually complete (99%). These tracer incubations show that isotope dilution can be significant in NH 4 + uptake experiments conducted with N‐rich macroalgae and that determination of 15 N atom % enrichment of the dissolved NH 4 + is recommended to avoid poor isotope recovery and underestimation of uptake rates.