Comparative rapid ammonium uptake by four species of marine phytoplankton1

A series of 15NH4+ uptake studies with four species of marine phytoplankton, Dunaliella tertiolecta (Dun), Phaeodactylum tricornutum (TFX‐1), Chaetoceros simplex (BBsm), and Thalassiosira weissflogii (Actin), maintained in NH4+‐limited continuous cultures at varying specific growth rates (µ) and temperatures, was done to examine the time‐dependency of NH4+ uptake and the relationship between physiological state and NH4+ uptake. The effect of growth rate on short term NH4+ uptake varied tremendously with species, but the trend of increasing V′M: µ̂ with decreasing relative growth rate (µ:µ̂) was demonstrated by all species down to µ ≃ 0.25% of µ̂. The diatoms, especially, were capable of sustaining a high V′M:µ̂ even at very high relative growth rates. Growth rate also influenced the time‐course of NH4+ uptake: for cells grown at ≈ 0.20–0.35 µ̂ an enhancement in NH4+ uptake was observed in the first 5 min of incubation, but not for cells at >0.90 µ̂. These data are consistent with the hypothesis that phytoplankton may be able to exploit short‐lived nutrient micropatches and may be growing at relatively rapid rates in oceanic waters.