RAPID AMMONIUM‐ AND NITRATE‐INDUCED PERTURBATIONS TO CHL <i>a</i> FLUORESCENCE IN NITROGEN‐STRESSED <i>DUNALIELLA TERTIOLECTA</i> (CHLOROPHYTA)¹

Abstract

When NH 4 + or NO 3 − was supplied to NO 3 − ‐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 NO 3 − and 10 μM for NH 4 + . Changes in fluorescence responses were characterized in D. tertiolecta both over 5 days of N starvation and in cells cultured at a range of NO 3 − ‐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 NO 3 − versus NH 4 + and were correlated with changes in maximum N uptake rate during N starvation. In severely N‐starved cells, the major fluorescence response to NO 3 − disappeared, whereas the response to NH 4 + persisted. N‐starved cells previously grown with NH 4 + alone showed fluorescence responses with NH 4 + but not NO 3 − additions. The distinct responses to NO 3 − and NH 4 + 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 NO 3 − or NH 4 + as an N source to phytoplankton populations and as a diagnostic tool for N limitation.