Nitrogen assimilation following NH4+ pulses in the red alga Gracilariopsis lemaneiformis: effect on C metabolism Juan J. V e r g a r a l , Kimon T. ~i r d~, F. X. ~i e l l~ ABSTRACT: The short-term effect of external NH,' 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 NH,' 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 NH,+ 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 NH,+ assimilation at a non-saturating irradiance for photosynthesls of 80 pm01 m-2 S-'. 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 prote~ns) during transient NH, + assimilation.

A dynamic model has been developed describing the effects of transient N assimilation following NH,+pulses on protein synthesis and on C mobilization in red algae. The model simulations indicate that the differential response of phycobiliproteins to N availability seems to be related to a more general response of chloroplast proteins to N supply. The model displays a high robustness. The incorporation of different functions of amino acid transport between the chloroplast and cytosol fractions, as well as different initial distributions of amino aclds between these fractions, has little effect on N incorporation at the protein level, with chloroplast proteins being much more affected than cytosolic ones by the variation of the external forcing function, the NH,' supply. With respect to cell C metabolism, the main changes promoted by a transient NH,+assimilation were not in total cell C but in the allocation of C between C reserve structures (carbohydrates) and organic N compounds (amino acids and proteins). The stoich~ometry of 6 C molecules needed per N molecule assimilated seems to be crucial in determining the rate of C mobilization in response to transient N assimilation. The development of the model provides further insights in the mechanism of C-N interaction In marine red algae, where the presence of particular N compounds such as phycoblllproteins and of C compounds such as cell wall polysaccharides and floridean starch is different compared to green algae and higher plants. The results of the simulations compared favorably wlth the experimental data reported for the red alga Gracilariopsis lemaneiformis.