ABSTRACT: 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 umol 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.

KEY WORDS: Ammonia . Nitrogen assimilation . Carbon . Carbohydrates . Phycobiliproteins . Gracilariopsis . Rhodophyta

Published in MEPS Vol. 122 (1995) on June 15
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1995