NITROGEN ASSIMILATION FOLLOWING NH4+ PULSES IN THE RED ALGA GRACILARIOPSIS-LEMANEIFORMIS - EFFECT ON C-METABOLISM

The short-term effect of external NH4+ availability on N assimilation, as well as on C metabolism, has been studied in the red alga Gracilariopsis lemaneiformis. This agarophyte seaweed showed a high capacity to take up external NH4+ and to channel it toward proteins. As in a previous study in Corallina elongata, it appears that N limitation redirected the flow of internal N toward nonpigmented proteins, whereas phycobiliprotein synthesis was preferentially stimulated with respect to other proteins in response to external NH4+ pulses. A possible mechanism by which chloroplastic protein synthesis is limited by external N availability more than cytosolic proteins could be involved. In relation to C metabolism, insoluble and soluble carbohydrates were mobilized in response to NH4+ assimilation at a non-saturating irradiance for photosynthesis of 80 mu mol m(-2) s(-1). The main change in C metabolism was not seen for total C. The principal effect was the flow of C among different compounds, from carbohydrates to organic N compounds (amino acids and proteins) during transient NH4+ assimilation.