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Nitrogen Metabolism of the Marine Microalga Chlorella autotrophica¹

Department of Botany, University of Toronto, Toronto, Ontario M5S 1A1 Canada

The levels of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) in Chlorella autotrophica (clone 580) are strongly regulated by the nitrogen source and salt concentration of the medium. GS is present at high levels in NO₃^–-grown cells, and at maximum levels in nitrogen-starved cells. However, the levels of GS in these cells are somewhat decreased by increasing salinity. Cells growing on NH₄⁺ have high NADPH-GDH activity, the levels of which increase with increasing NH₄⁺ supply, while GS decreases to a very low level under these conditions. Salinity intensifies the induction of NADPH-GDH activity in NH₄⁺-grown cells. The levels of NADH-GDH are low in this alga, but present under all growth conditions. Methionine sulfoximine (MSX) has little effect on growth and nitrogen assimilation of the alga in the presence of NH₄⁺.