AME 61:89-104 (2010)  -  DOI:

Nitrogen uptake by phytoplankton and bacteria during an induced Phaeocystis pouchetii bloom, measured using size fractionation and flow cytometric sorting

P. B. Bradley1,7,*, M. P. Sanderson1, J. C. Nejstgaard2,3, A. F. Sazhin4, M. E. Frischer5, L. M. Killberg-Thoreson1, P. G. Verity5,†, L. Campbell6, D. A. Bronk1

1Virginia Institute of Marine Science, The College of William & Mary, PO Box 1346, Gloucester Point, Virginia 23062, USA
2Uni Environment, Thormohlensgate 49B, 5020 Bergen, Norway
3Department of Biology, University of Bergen, Thormohlensgate 53A, 5006 Bergen, Norway
4P.P. Shirshov Institute of Oceanology RAS, 36 Nakhimovsky Prospect, Moscow 117997, Russia
5Skidaway Institute of Oceanography, 10 Ocean Science Circle, Savannah, Georgia 31411, USA
6Department of Oceanography, Texas A&M University, College Station, Texas 77843, USA
7Present address: National Oceanic and Atmospheric Administration, 1305 East West Hwy N/MB5, Silver Spring, Maryland 20910, USA

ABSTRACT: Uptake of inorganic and organic nitrogen (N) by phytoplankton and bacteria was investigated during a mesocosm study conducted in Raunefjord, Norway in April 2005. One mesocosm was fertilized with nitrate and phosphate at a ratio of 16:1 and maintained in the light, while one unamended light mesocosm served as a control. Dissolved nutrients, phytoplankton and bacterial biomass, and phytoplankton community composition were monitored throughout the 26 d experiment. Uptake of 15N-labeled ammonium and nitrate, and dual-labeled (15N and 13C) urea and dissolved free amino acids (DFAA) was measured for phytoplankton and bacteria using 2 methods: size fractionation into 0.2–0.8 and >0.8 µm size classes, and flow cytometric sorting based on chlorophyll autofluorescence. Prior to fertilization, dissolved inorganic N concentrations were low and comprised ~5% of total dissolved N. Added nitrate was completely utilized in the amended mesocosm within 10 d, stimulating a large bloom of colonial Phaeocystis pouchetii. Ammonium contributed over half of total measured N uptake by phytoplankton and bacteria in both enclosures, while nitrate and urea each supplied roughly 10 to 25%. Overall, DFAA were a negligible N source to phytoplankton but contributed 11% to total bacterial N uptake. Bacterial uptake represented a significant portion of total uptake of all N forms, especially urea and DFAA. Comparison of the 2 methods for measuring phytoplankton versus bacterial uptake demonstrates how the use of 0.8 µm filters can lead to significant overestimation of phytoplankton N uptake due to the retention of bacterial biomass.

KEY WORDS: Nitrogen uptake · Mesocosm experiments · Phaeocystis · Phytoplankton · Bacteria · Flow cytometry

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Cite this article as: Bradley PB, Sanderson MP, Nejstgaard JC, Sazhin AF and others (2010) Nitrogen uptake by phytoplankton and bacteria during an induced Phaeocystis pouchetii bloom, measured using size fractionation and flow cytometric sorting. Aquat Microb Ecol 61:89-104.

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