Combined Nitrogen

Algae can use a wide range of combined N sources. All of them can use NH4+, and probably also use urea and one of more amino acids; most of them can also use NO2- and NO3-, and some can use betaines and/or purines. Transport of combined N into cells very often uses H+ or Na+ symport. Two cations transported per anionic N species, and one cation transported per neutral N species, enables the electrical potential difference across the membrane generated by active cation efflux to be used to increase the accumulation ratio inside:outside of the combined N species. Cationic N forms, e.g. NH4+, sometimes occur at very low concentrations in the natural environment, and cation symport can increase the steady-state NH4+ concentration. The transporters have been to some extent characterised at the molecular level, especially for the plasmalemma. Assimilation of inorganic N species into organic N within the cell use well-established pathways, i.e. NO3- reductase, NO2- reductase, and glutamine synthetase – glutamine-oxoglutarate aminotransferase enzymes. N assimilation, and especially the initial step (NO3- reductase), are under more direct redox control in microalgae than in vascular plants. Combined N species which as NO and NO3- are involved in signalling within the cell, but extent to which they modulate metabolism in response to internal and external cues needs clarification. It is important to bear in mind that the conclusions drawn here generally come from work on relatively few microalgal species, and generalisations should be viewed with caution.