Modeling Nitrate Uptake in Freshwater Phytoplankton

Karl Schuchard, Ursula Sicking, Stephanie Donaldson

Abstract


Freshwater phytoplankton are typically limited by nitrogen or phosphorus. Additions of these nutrientscan lead to blooms, so understanding nutrient metabolism is important for predicting ecosystem responses.Nitrate metabolism was studied in an urban pond and large lake using controlled incubations of whole-water cultures. Cultures were either not enriched, or enriched with phosphate, and nitrate or ammoniumand followed for two weeks, measuring nutrients, chlorophyll, particulate C and N, the activity of a keyenzyme, nitrate reductase (NR), and kinetics of nitrate uptake and NR activity. Pond phytoplankton re-sponded rapidly to enrichments; added nitrate disappeared in 5 d. Lake phytoplankton showed slowergrowth, with nitrate and phosphate depleted over 1-2 weeks. Only nitrate-enriched cultures had NR ac-tivity and it disappeared when nitrate was exhausted or when cells stopped growing. Laboratory nitrateuptake showed novel kinetics; at low external nitrate concentrations uptake rates increased to a plateaubut also showed a linear increase in uptake rate with much higher nitrate concentrations. Half saturationconstants for uptake were higher ( 20 M) than for NR activity ( 1 M), suggesting minimal developmentof intracellular nitrate pools. Ongoing work includes identication of algal taxa in the communities us-ing 18S rRNA gene sequencing and matching to databases. An agent based model (i.e. a model in whichparameters are allowed to vary among individuals) was also developed to examine if phytoplankton com-munity responses to nitrate can be predicted from individual species characteristics. The model was devel-oped in Netlogo using Michaelis-Menten kinetic equations for nitrate uptake, and intracellular assimilationthrough nitrate reduction by NR activity. Ranges of kinetic parameters for cells were selected from ourlaboratory measurements, and literature values for species. A key component of the model is a cellular Ncontent threshold for cell division.

Keywords


Plytoplankton, Nutrients, Modeling

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