Plant Physiol. Drug Metab Dispos
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Plant Physiology 94:1124-1130 (1990)
© 1990 American Society of Plant Biologists

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Metabolism and Enzymology

Anaerobic Metabolism in the N-Limited Green Alga Selenastrum minutum1

II. Assimilation of Ammonium by Anaerobic Cells

Greg C. Vanlerberghe and David H. Turpin

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

The green alga Selenastrum minutum (Naeg.) Collins is able to assimilate NH4+ in the dark under anaerobic conditions (GC Vanlerberghe, AK Horsey, HG Weger, DH Turpin [1989] Plant Physiol 91: 1551-1557). In the present study, analysis of metabolites following addition of NH4+ to cells acclimated to anaerobic conditions has shown the following. There was a transient decline in adenylate energy charge from 0.6 to 0.4 followed by a recovery back to ~0.6. This was accompanied by a rapid increase in pyruvate/phosphoenolpyruvate and fructose-1,6-bisphosphate/fructose-6-phosphate ratios indicating activation of pyruvate kinase and 6-phosphofructokinase, respectively. There was also an increase in fructose-2,6-bisphosphate, which, since this alga lacks pyrophosphate dependent 6-phosphofructokinase can be inferred to inhibit gluconeogenic fructose-1,6-bisphosphatase. These changes resulted in an increase in the rate of anaerobic starch breakdown. Anaerobic NH4+ assimilation also resulted in a two-fold increase in the rate of production of the major fermentative end-products in this alga, D-lactate and ethanol. There was no change in the rate of accumulation of the fermentative end product succinate but malate accumulated under anoxia during NH4+ assimilation. A rapid increase in Gln and decline in Glu indicates that primary NH4+ assimilation under anoxia was via glutamine synthetase-glutamate synthase. Almost all N assimilated under these conditions was sequestered in alanine. These results allow us to propose a model for the regulation of carbon metabolism during anaerobic NH4+ assimilation.

1 Supported by the Natural Sciences and Engineering Research Council of Canada. G. C. V. acknowledges support from a NSERC postgraduate scholarship and a R. S. McLaughlin Fellowship from Queen's University.






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Copyright © 1990 by the American Society of Plant Biologists