GLUTAMINE-SYNTHETASE IN THE MARINE COCCOLITHOPHORID EMILIANIA-HUXLEYI(PRYMNESIOPHYCEAE) - REGULATION OF ACTIVITY IN RELATION TO LIGHT AND NITROGEN AVAILABILITY

Regulation of glutamine synthetase (GS; EC 6.3.1.2) in relation to lig ht and nitrogen availability was investigated in Emiliania huxleyi. GS activity was low when effective illumination of the cells, and so pho tosynthetic activity, was high whereas it was high when effective illu mination 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 p hase. Nitrogen starvation stimulated GS activity and also had an effec t on the kinetic properties of GS. The apparent K-m of GS for NH4+ was 1.05 mM when cells were grown on NO3- and 0.146 mM when cells were ni trogen-starved. Subsequent to the addition of 1 mM NH4+ to a NO3--grow n culture at the beginning of the exponential phase, GS was progressiv ely inactivated, whereas NADPH-GDH activity largely increased. On the other hand, GS remained active and NADPH-GDH not detectable when ammon ium ions were added during the mid-exponential phase. Algae were able to grow in the presence of 2 mM MSX, with NO3- as the nitrogen source, though GS was largely inactivated under these conditions. Meanwhile, NADPH-GDH activity doubled. Nitrogen and carbon metabolism appear to b e intimately linked in the regulation of Emiliania GS activity. The re sults also suggest a major role for GS in ammonium assimilation in Emi liania huxleyi. Copyright (C) 1997 Elsevier Science Ireland Ltd.