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Metabolism and Enzymology

Effects of Phosphorus Limitation on Respiratory Metabolism in the Green Alga Selenastrum minutum¹

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

The effects of phosphorus nutrition on several physiological and biochemical parameters of the green alga, Selenastrum minutum, have been examined. Algal cells were cultured in chemostats under conditions of either Pi limitation or nutrient sufficiency. Pi limitation resulted in: (a) a 5-fold lower rate of respiration, (b) a 3-fold decline in rates of photosynthetic carbon dioxide fixation and oxygen evolution, (c) a 3-fold higher rate of dark carbon dioxide fixation, (d) significant increases in activities of phosphoenolpyruvate (PEP) carboxylase and PEP phosphatase (128% and 158% of nutrient sufficient activities, respectively), (e) significant reductions in activities of nonphosphorylating NADP-glyceraldehyde-3-phosphate dehydrogenase and NAD malic enzyme, and (f) no change in levels of ATP:fructose-6-phosphate 1-phosphotransferase, phosphorylating NAD-glyceraldehyde-3-phosphate dehydrogenase, 3-phosphoglycerate kinase, and pyruvate kinase. The intracellular concentrations of Pi, ATP, AMP, soluble protein, and chlorophyll were also significantly reduced in response to Pi limitation. As well, the level of ADP was about 11-fold lower in the Pi-limited cells as compared to the nutrient sufficient controls. It was predicted that because of this low level of ADP, pyruvate kinase catalyzed conversion of PEP to pyruvate may be restricted in Pi-limited cells. During Pi limitation, PEP carboxylase and PEP phosphatase may function to "bypass" the ADP dependent pyruvate kinase, as well as to recycle Pi for its reassimilation into cellular metabolism.

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