Nitrogen metabolism and activity of amino acid metabolizing enzymes in the unicellular green alga Dunaliella sp. under long-term salinity and arginine treatment.

The accumulation of some free amino acids and their metabolism were investigated in Dunaliella sp. cells adapted to three salt levels (1, 2, and 3 M NaCl) in the presence or absence of 5 mM Arginine (Arg) to assess the relationship between salinity tolerance and nitrogen metabolism. Salt-adapted cells showed a large decrease in density depending on salt concentration but accumulated high concentrations of free amino acids such as proline, Arg, and γ-aminobutyric acid (GABA). Salinity increased the activities of enzymes involved in nitrogen metabolism, including nitrate reductase (NR), glutamine synthetase (GS), and glutamate synthase (NADH-GOGAT), as well as several amino acid metabolizing enzymes such as arginase, arginine decarboxylase (ADC), ornithine aminotransferase (OAT), glutamate dehydrogenase (GDH), and glutamate decarboxylase (GDC). Arg treatment improved cell density at suspensions of 1 and 2 M NaCl, which was associated with a high accumulation in glutamate and GABA and decreased Arg and proline. In contrast, Arg and GABA accumulated in Arg-treated cells at 3 M NaCl, without changes in proline and glutamate levels. Accumulation of GABA in Arg-treated cells occurred while GDC activity decreased or remained unchanged. Like amino acids, the activities of all enzymes involved in amino acid metabolization were differently changed by the Arg treatment. These results favor a determining role for nitrogen metabolism in tolerance of Dunaliella sp. cells to long-term salinity. Arg treatment balances the metabolism of carbon and nitrogen, perhaps by different modifying the activity of nitrogen and amino acid metabolizing enzymes and operating the biosynthesis of signaling molecules. [ABSTRACT FROM AUTHOR]