MEPS 123:245-258 (1995)  -  doi:10.3354/meps123245

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

Lipschultz F

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 nitrogen-specific 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 (muN, time-1) and absolute uptake rates (mass volume-1 time-1) of ammonium were greater for small (3-10 um) phytoplankton than for larger (10-53 um) 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 (<3 um) particles that were removed prior to sorting: phytoplankton represented only 50% of the total rate. In contrast, phytoplankton uptake of nitrate represented only about one-third of the total uptake, and the remaining fractions contributed only 10%, leaving about 60% unaccounted for. Future applications of flow cytometry with stable isotopes could be used to investigate diel variations in muN and muC (carbon-specific uptake rates) and provide an estimate of phytoplankton growth rates.


Flow cytometry . Nitrogen . Phytoplankton . Assimilation . Growth rates


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