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.
- Chlorophyll a fluorescence
- Nitrogen limitation