Regulation of Nitrogen Assimilation in Green Microalgae

Abstract

Green 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 NADP-GDH exclusively in the light. Reassimilation of ammonium released via photorespiration—a process much less important in green microalgae than in higher plants—likely occurs via a chloroplast-located GS/GOGAT cycle. Nitrate and nitrite reductases are strongly regulated by light conditions and the nitrogen status of the cells. The different abilities of green microalgae to utilize organic nitrogen compounds are due to differences in availability of transport systems and catabolic enzyme machinery.

Keywords:

• Ammonium
• glutamate dehydrogenase
• glutamate synthase
• glutamine synthetase
• green microalgae
• heterotrophy
• nitrate
• nitrite
• nitrogen assimilation
• organic nitrogen
• photoautotrophy
• photorespiration
• regulation
• transport mechanisms
• urea