A dynamic model of transient NH4+ assimilation in red algae

Abstract

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.

Allometric evidence for the dominant role of surface cells in ammonium metabolism and photosynthesis in northeastern New Zealand seaweeds

“…If GS2 is the isoform involved in primary nitrogen assimilation (Oliveira et al 1997), then this could explain the association of ammonium assimilation with the outer, photosynthetic cells. This explanation is particularly compelling given the strong coupling between ammonium assimilation and carbon metabolism in algae (Turpin 1991, Vergara et al 1997.…”

“…While this difference could be due to predominantly diffusive uptake in New Zealand seaweeds (Taylor et al 1998), it is also possible that other surface-related processes, such as photosynthesis, play an important role. Photosynthesis and nitrogen metabolism are integrally coupled in algae (Turpin et al 1988, Vergara et al 1997, with the active uptake and assimilation of ammonium dependent on photogenerated ATP and reductant (Syrett 1981, Turpin 1991. Moreover, the assimilation of ammonium also requires carbon intermediates from the respiratory tricarboxylic acid cycle.…”

The role of hydrodynamics in structuring in situ ammonium uptake within a submerged macrophyte community

“…Ammonium (NH 4 + ) uptake by submerged macrophyte communities is a major pathway by which nitrogen is removed from the water column and potentially retained within shallow, marine, benthic ecosystems (Wallentinus 1984; Touchette and Burkholder 2000; Fernandes et al 2009). The high foliar NH 4 + uptake capacity of many marine angiosperms (Touchette and Burkholder 2000; Rubio et al 2007; Alexandre et al 2011) and macroalgae (O'Brien and Wheeler 1987; Vergara et al 1997; Malta et al 2005) indicates that, in systems with low‐to‐moderate nutrient concentrations, uptake is often controlled by rates of transfer across concentration boundary layers, rather than by enzymatic processes (Bilger and Atkinson 1992). Hence, an increase in NH 4 + concentration or water velocity strongly enhances macrophyte uptake (Hurd et al 1996; Thomas et al 2000; Koch et al 2006).…”