Regulation of Nitrogen Assimilation in Green Microalgae

Abstract

AbstractGreen microalgae have evolved a diversity of regulatory mechanisms for assimilating inorganic and organic nitrogen compounds. Ammonium, which is a preferred nitrogen source under most conditions, is usually assimilated via the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle. However, some green microalgae assimilate ammonium via NADP-dependent glutamate dehydrogenase (GDH) under high ammonium conditions (e.g., Chlorella species), or under conditions of restricted nitrogen supply (Stichococcus bacillaris, which possesses a high-affinity NADP-GDH). NAD-dependent glutamate dehydrogenase appears to function mainly under heterotrophic conditions, and when amino acids serve as sources of nitrogen, thus suggesting a catabolic role for this enzyme. In S. bacillaris, when ammonium assimilation occurs chiefly via the GS/GOGAT cycle, GS1 (cytosolic isozyme) appears to be responsible for glutamine synthesis in the dark, and GS2 (chloroplastic isozyme) in the light. S. bacillaris synthesizes GS2 and N...