Rapid ammonium- and nitrate-induced perturbations to chl a fluorescence in nitrogen-stressed Dunaliella tertiolecta (Chlorophyta)

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 muM for NO3 (-) and 10 muM 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.