Glutamine synthetase in the marine coccolithophorid Emiliania huxleyi (Prymnesiophyceae): regulation of activity in relation to light and nitrogen availability

Abstract

Regulation of glutamine synthetase (GS; EC 6.3.1.2) in relation to light and nitrogen availability was investigated in Emiliania huxleyi. GS activity was low when effective illumination of the cells, and so photosynthetic activity, was high whereas it was high when effective illumination was lower. Measurements of glutamate dehydrogenase (GDH; EC 1.4.1.3) activities in the same type of culture revealed that NADPH-GDH was only present at a low level at the beginning of the exponential phase. Nitrogen starvation stimulated GS activity and also had an effect on the kinetic properties of GS. The apparent K m of GS for NH 4 + was 1.05 mM when cells were grown on NO 3 − and 0.146 mM when cells were nitrogen-starved. Subsequent to the addition of 1 mM NH 4 + to a NO 3 −-grown culture at the beginning of the exponential phase, GS was progressively inactivated, whereas NADPH-GDH activity largely increased. On the other hand, GS remained active and NADPH-GDH not detectable when ammonium ions were added during the mid-exponential phase. Algae were able to grow in the presence of 2 mM MSX, with NO 3 − as the nitrogen source, though GS was largely inactivated under these conditions. Meanwhile, NADPH-GDH activity doubled. Nitrogen and carbon metabolism appear to be intimately linked in the regulation of Emiliania GS activity. The results also suggest a major role for GS in ammonium assimilation in Emiliania huxleyi.