AMMONIUM RHYTHM IN CULTURES OF THE CYANOBACTERIUM MICROCYSTIS-FIRMA

Over a period of several days, rhythmic changes in extracellular NH4concentration take place in cultures of the cyanobacterium Microcystis firma (Breb et Lenorm.) Schmidle, strain Gromov/St. Petersb. 398, und er conditions of restricted CO2 supply and light/dark alternation. The changes are enhanced by nitrate supply. Among the various processes g enerating intracellular NH4+ (NH4+ uptake, NO3- reduction, protein and amino acid degradation, photorespiration), NO3- reduction appears as the one most important. This can be concluded from experiments with an d without nitrate and/or ammonium in the medium. In the presence of sa turating CO2, continuous light, or continuous darkness, rhythmic NH4oscillations are not induced. Studies of the incorporation of NH4+ nit rogen by in vivo N-15-NMR show that if CO2 is supplied, N-15 is accumu lated in several components with the following time course: in the fir st hour in Gln (delta), in the second hour in the alpha-amino groups o f most nonbranched amino acids, in the third hour in gamma-aminobutyri c acid (GABA), Orn (delta) and Lys (epsilon), and in the sixth hour in Ala. Carbon limitation, however, results in accumulation of label in the amide nitrogen of glutamine only.