Effect of ammonium on nitrate utilization by Emiliania huxleyi, a coccolithophore from the oceanic northeastern Pacific

Explanations for the high nitrate (NO3-), low phytoplankton biomass of the subarctic NE Pacific have mainly centered on iron limitation and grazing pressure. Although ammonium (NH4+) has been proven to inhibit NO3- uptake by phytoplankton in numerous laboratory and field studies, this hypothesis has not been experimentally tested for the subarctic Pacific. The effect of increasing NH4+ concentrations on NO3- uptake by Emiliania huxleyi, a coccolithophore isolated from Ocean Station Papa in the subarctic NE Pacific, is examined. The effect of diel periodicity on NO3- uptake and other physiological parameters for E. huxleyi during a 14:10 h light:dark cycle are also reported. About 84% of the total daily NO3- was taken up during the light period. Cell division occurred only during the last part of the dark period. Chlorophyll a, nitrogen, and carbon quotas and cell volume increased during the light period and decreased during the dark. The presence of NH4+ inhibited NO3- uptake in E. huxleyi. Nitrate uptake rates were reduced to half the maximum value at 0.24 mu M NH4+, and maximum realized inhibition was similar to 100% at 2.2 mu M NH4+. If this laboratory result is extrapolated to field conditions, the inhibition of NO3- uptake rates for the small size class of phytoplankton would be predicted to be 38 to 70% for the range of ambient NH4+ concentrations found in the oceanic NE Pacific.