A dynamic model of transient H4+ assimilation in red algae

Abstract

A dynamic model has been developed describing the effects of transient N assimilation following NH4+ 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 acids 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 NH4+ supply. With respect to cell C metabolism, the main changes promoted by a transient NH4+ 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 stoichiometry 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 phycobiliproteins 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 with the experimental data reported for the red alga Gracilariopsis lemaneiformis.