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

When NH₄ ⁺ or N_O3^- 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.