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Metabolism and Enzymology

Steady-State Chlorophyll a Fluorescence Transients during Ammonium Assimilation by the N-Limited Green Alga Selenastrum minutum¹

Department of Biology, Queen's University, Kingston, Ontario, Canada K7L 3N6

The assimilation of ammonium by the N-limited green alga Selenastrum minutum results in the suppression of photosynthetic electron flow from H₂O to CO₂ (6, 7, 18). In this study, results are presented which describe the correponding change in steady-state chlorophyll a fluorescence. The addition of ammonium resulted in a transient decline in fluorescence followed by a marked increase. Fluorescence did not return to control levels until the added ammonium had been assimilated. Analysis of the fluorescence transients showed that ammonium assimilation resulted in a rapid increase in nonphotochemical quenching (Q_e) peaking 10 to 15 seconds after ammonium addition. Q_e then decreased dramatically reaching a minimum value approximately 45 seconds following ammonium addition and returned to the control level only after the added ammonium had been assimilated. There were no effects of ammonium addition on photochemical quenching (Q_q) for approximately 10 to 15 seconds at which time both gross O₂ evolution (as measured by mass spectrometry) and Q_q declined. In the presence of D,L-glyceraldehyde or when cells were held at the CO₂ compensation point, the addition of ammonium resulted in a decline in Qe 10 to 15 seconds after addition. The Q_e peak and the Q_q decline were absent. These results imply that the transient increase in Q_e and the subsequent decline in Q_q may be attributed to the decline in Calvin cycle activity during ammonium assimilation. The decline in Q_e is apparently a direct result of ammonium assimilation. The observation that the Q_e peak precedes the Q_q decline would be consistent with the decreases in Calvin cycle carbon flow occurring at the kinase reactions prior to glyceraldehyde-3-phosphate dehydrogenase.