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 NH,' concentrations on NO< uptake by Emiliania huxleyi, a coccolithophore isolated from Ocean Station Papa in the subarctic NE Pacific, is examined.The effect of die1 periodicity on NO; uptake and other physiological Parameters for E. huxleyj during a 14:10 h 1ight:dark cycle are also reported.About 84 % of the total daily NO3-was taken up during the light period.Ce11 division occurred only d u n n g the last part of the dark penod.Chlorophyll a, nitrogen, and carbon quotas and cell volume increased dunng the light period and decreased during the dark.The presence of NH,+ inhibited NOYi-uptake in E. huxleyi.Nitrate uptake rates were reduced to half the maximum value at 0.24 pM YH,', and maximum realized inhibition was -100% at 2.2 FM NH,' 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 foiind in the oceanic NE Pacific KEY WORDS: Nitrate uptake .Ammonium inhibition .Die1