Nitrogen-specific uptake rates of marine phytoplankton isolated from natural populations of particles by flow cytometry

• Published in: Marine ecology. Progress series (Halstenbek) 1995, Vol.123 (1/3), p.245-258
• Main Author: Lipschultz, Fredric
• Format: Article
• Language: English
• Subjects: Bacteria; Chaetoceros; Chemical composition; Chlorophylls; Cytometry; Dunaliella tertiolecta; Filtration; Leptocylindrus; Marine; Nitrates; Nitrogen; Phytoplankton; Plankton; Quaternary ammonium compounds; Thalassiosira
• ISSN: 0171-8630; 1616-1599
• DOI: 10.3354/meps123245

Phytoplankton were isolated from natural, heterogeneous populations of particles in Boothbay Harbor, Maine, USA using flow cytometry with cell sorting to measure true rates of nitrogenspecific uptake of 15N without the bias due to detritus, bacteria or other heterotrophs that affects traditional filtration techniques. After incubation under light or dark conditions with 15NH4+ or 15NO3−, particles were either directly filtered or were sorted to isolate phytoplankton and then filtered for isotopic analysis. Nitrogen-specific (μN, time−1) and absolute uptake rates (mass volume−1 time−1) of ammonium were greater for small (3–10 μm) phytoplankton than for larger (10–53 μm) phytoplankton, whereas the opposite was observed for uptake of nitrate. Accounting for pronounced diel variations in uptake from both nitrogen sources, algal growth rates were calculated to be 0.38 d−1 on ammonium and 0.05 d−1 on nitrate. The absolute rate of NH4+ uptake by all particles as assayed by filtration was approximately balanced by the summed rates of the 2 sorted fractions and the small (