Dark Ammonium Assimilation Reduces the Plastoquinone Pool of Photosystem 11 in the Green Alga Selenastrum minutumI

The impact of dark NH4+ and N03assimilation on photosynthetic light harvesting capability of the green alga Selenastrum minutum was monitored by chlorophyll a fluorescence analysis. When cells assimilated NH4+, they exhibited a large decline in the variable fluorescence/maximum fluorescence ratio, the fluorescence yield of photosystem 11 relative to that of photosystem I at 77 kelvin, and 02 evolution rate. NH4+ assimilation therefore poised the cells in a less efficient state for photosystem II. The analysis of complementary area of fluorescence induction curve and the pattern of fluorescence decay upon microsecond saturating flash, indicators of redox state of plastoquinone (PQ) pool and dark reoxidation of primary quinone electron acceptor (QA), respectively, revealed that the PQ pool became reduced during dark NH4+ assimilation. NH4+ assimilation also caused an increase in the NADPH/NADP+ ratio due to the NH4+ induced increase in respiratory carbon oxidation. The change in cellular reductant is suggested to be responsible for the reduction of the P0 pool and provide a mechanism by which the metabolic demands of NH4+ assimilation may alter the efficiency of photosynthetic light harvesting. N03assimilation did not cause a reduction in PQ and did not affect the efficiency of light harvesting. These results illustrate the role of cellular metabolism in the modulating photosynthetic processes. tation energy favoring PSI. These authors (22) suggested that this alteration aids in balancing the higher ATP/NAD(P)H requirement of NH4' assimilation relative to CO2 fixation through PSI-supported cyclic photophosphorylation. N03assimilation, on the other hand, requires more NAD(P)H relative to ATP for its assimilation than NH4', and did not cause a suppression in 02 evolution nor introduce any alteration in fluorescence characteristics of PSII or PSI (3, 4, 8, 22, 23). NH4' assimilation in light influences the respiratory and photosynthetic carbon metabolism in an integrated fashion (18, 21, 24). In dark, N-limited S. minutum cells are capable of assimilating NH4' at a rate equivalent to that in light (21, 24). Dark NH4' assimilation influences carbon metabolism by stimulating glycolysis and mitochondrial respiration and enhancing dark anaplerotic CO2 fixation about 40-fold (24, 25). The impact of these metabolic alterations on subsequent photosynthetic behavior is not known. Understanding the interplay of photosynthetic, respiratory, and N metabolism requires an assessment on the effect ofdark NH4' assimilation on photochemical functions. The present investigation reveals how NH4' assimilation in the dark poises S. minutum in a low efficiency photochemical state for PSII (state 2) and illustrates the regulatory effects of N assimilation and respiration on the light utilization processes of photosynthesis. The assimilation of NH4' or NO3by N-limited cells of Selenastrum minutum occurs at such high rates that photosynthetic carbon fixation does not meet the carbon demands for amino acid synthesis (14, 19). Under these conditions, photosynthetic carbon fixation is suppressed (3, 4), and the carbon demands are met by starch degradation (14, 19). During NH4' assimilation, the suppression of photosynthetic carbon fixation removes the sink for photogenerated reductant and causes a decline in gross 02 evolution (for a review, see ref. 21). Chl a fluorescence analysis, a sensitive indicator of photosynthetic processes, revealed that NH4' assimilation also results in a decline in the photochemical yield of PSII (8, 23). Recently, Turpin and Bruce (22) observed that NH4' assimilation by S. minutum caused a redistribution of exciSupported by the Natural Sciences and Engineering Research Council grants to D.H.T. and D.B. Current address: Department of Botany, University of British Columbia, Vancouver, B.C., V6TlW5. MATERIALS AND METHODS