ABSTRACIrSynechococcus kopoliensis was grown over a wide range of dissolved inorganic carbon (DIC) concentrations (4-25,000 micromolar) which were obtained by varying culture pH (6.2-9.6)

The effects of ammonium assimilation on photosynthetic carbon fixation and 02 exchange were examined in two species of N-limited green algae, Chlorella pyrenoidosa and Selenastrum minutum. Under light-saturating conditions, ammonium assimilation resulted in a suppression of photosynthetic carbon fixation by S. minutum but not by C. pyrenoidosa. These different responses are due to different relationships between cellular ribulose bisphosphate (RuBP) concentration and the RuBP binding site density of ribulose bisphosphate carboxylase/oxygenase (Rubisco). In both species, ammonium assimilation resulted in a decrease in RuBP concentration. In S. minutum the concentration fell below the RuBP binding site density of Rubisco, indicating RuBP limitation of carboxylation. In contrast, RuBP concentration remained above the binding site density in C. pyrenoidosa. Compromising RuBP regeneration in C. pyrenoidosa with low light resulted in an ammonium-induced decrease in RuBP concentration below the RuBP binding site density of Rubisco. This resulted in a decrease in photosynthetic carbon fixation. In both species, ammonium assimilation resulted in a larger decrease in net 02 evolution than in carbon fixation. Mass spectrometric analysis shows this to be a result of an increase in the rate of mitochondrial respiration in the light.The assimilation of ammonium by photosynthetic organisms results in changes to both photosynthetic and respiratory carbon metabolism. This is illustrated by the diversion of recent photosynthate from the synthesis of starch to the production of TCA cycle2 intermediates. The keto-acids thus produced are then available for assimilation of ammonium into amino acids (1, 9, 31). There is evidence that in natural environments algal growth may be limited by inorganic nitrogen (7). A key adaptation under these conditions is an increase in the saturated rate of nitrogen uptake and assimilation (29). It is therefore not surprising that the supply of nitrogen to N-limited algae affects photosynthetic and respiratory metabolism to a much greater extent than in Nsufficient algae (8,16,25).In some species of N-limited microalgae, the rapid assimilation (5, 6, 17-20, 22, 26, 27). In a study with the N-limited green alga Selenastrum minutum (8), it was shown that the suppression of photosynthetic carbon fixation during the assimilation of inorganic nitrogen coincided with a decrease in RuBP concentration. This implied that RuBP levels may be responsible for limiting carbon fixation. In Chlorella pyrenoidosa, however, nitrogen assimilation results in large decreases in RuBP with little or no effect on photosynthetic carbon fixation (12). Given this apparent discrepancy, the role of RuBP in determining the rate of photosynthetic carbon fixation during N-assimilation requires further clarification.Net photosynthetic 02 evolution also exhibits a wide range of responses to N assimilation. In some N-limited algae, N assimilation results in only a slight suppression of net 02 evolution (10), while in others it ...

To examine the factors which limit photosynthesis and their role in photosynthetic adaptation to growth at low dissolved inorganic carbon (DIC) ,