| Abstract: | When NH4 + or NO3 ? was supplied to NO3 ? ‐stressed cells of the microalga Dunaliella tertiolecta Butcher, immediate transient changes in chl a fluorescence were observed over several minutes that were not seen in N‐replete cells. These changes were predominantly due to nonphotochemical fluorescence quenching. Fluorescence changes were accompanied by changes in photosynthetic oxygen evolution, indicating interactions between photosynthesis and N assimilation. The magnitude of the fluorescence change showed a Michaelis‐Menten relationship with half‐saturation concentration of 0.5 μM for NO3 ? and 10 μM for NH4 + . Changes in fluorescence responses were characterized in D. tertiolecta both over 5 days of N starvation and in cells cultured at a range of NO3 ? ‐limited growth rates. Variation in responses was more marked in starved than in limited cells. During N starvation, the timing and onset of the fluorescence responses were different for NO3 ? versus NH4 + and were correlated with changes in maximum N uptake rate during N starvation. In severely N‐starved cells, the major fluorescence response to NO3 ? disappeared, whereas the response to NH4 + persisted. N‐starved cells previously grown with NH4 + alone showed fluorescence responses with NH4 + but not NO3 ? additions. The distinct responses to NO3 ? and NH4 + may be due to the differences between regulation of the uptake mechanisms for the two N sources during N starvation. This method offers potential for assessing the importance of NO3 ? or NH4 + as an N source to phytoplankton populations and as a diagnostic tool for N limitation. |
