Abstract In Oscillatoria chalybea, as in other cyanobacteria, nitrate reduction depends directly on photosynthetic activity. Hence, nitrate reduction occurs only in the light and appears inhibited when photosynthesis is inhibited by the herbicide DCMU . Growth of Oscillatoria cells is possible on a variety of exogenous nitrogen sources in the medium and appears largely independent on the type of nitrogen source. However, if citrulline is the exogenous nitrogen source or if no exogenous nitrogen source is given in the medium, growth appears almost fully inhibited. Nitrate reductase activity measured in French-press particles of nitrate-grown cells is dependent on the age of the culture with maximum nitrate reductase activity being reached on the 5th day. Thereafter activity decreases steeply to less than 20% of the maximal activity within 10 days. Besides the growth stage it is the type of exogenous nitrogen source used in the medium which is important for the development of nitrate reductase activity. It appears that in the presence of nitrate, nitrite and arginine, nitrate reductase activity is induced whereas in the presence of ammonia or amino acids like alanine nitrate reductase activity is not induced, as already reported in the literature. Nitrate reductase is also induced if arginine and ammonia are simultaneously offered as exogenous nitrogen source. Arginine metabolism in Oscillatoria cells is characterized by the fact that thylakoid preparations of Oscillatoria catalyze the trans formation of arginine to give ornithine and ammonium. The arginine-metabolizing enzyme differs from the usual arginine-induced arginase. The enzyme seems to be constitutive, not manganese-dependent, exhibiting an approximately 5 times higher substrate affinity to arginine than the known arginase. In the present paper we propose that in Oscillatoria it is arginine which induces the synthesis of nitrate reductase.
