SUMMARY: Mg-limited growth of the cyanobacterium was investigated in batch and chemostat cultures. In batch cultures the growth rate of the organism depended on the Mg concentration up to 5 μ. Although the maximum growth rate was achieved at this concentration, the organism formed aseptate filaments of three to four times the‘normal’ cell length. About 90 min after increasing the Mg concentration from 5 μ to 1 m the cell size decreased, followed by an increase in the division rate, which lasted for about 60 min and resulted in a 66% increase in cell number. The rates of DNA, RNA and protein synthesis were not altered during these Mg shift-up experiments, showing that the control by Mg of growth had been separated from its control of cell division, In Mg-limited chemostat cultures, the mean cell volume decreased from about 2.0 to 0.6 μm when the Mg concentration was increased from 2.5 to 10 μ. This increase in Mg also resulted in an increase in the calculated intracellular Mg concentration from 27 to 78 m, and the amount of cellular Mg bound in chlorophyll increased from 17 to 22%. A comparison of Mg- and SO -limited chemostat cultures showed that the mean cell volume decreased with increasing dilution rate when Mg was the limiting factor, whereas it increased with dilution rate when SO was limiting. Only small differences in the rates of RNA and protein synthesis were found in the two cultures, although the synthesis of RNA was Mg-dependent. The ratio of total RNA to protein, which gives the amount of RNA necessary to synthesize one protein unit (RNA efficiency), was independent of the growth rate in both SO - and Mg-limited chemostat cultures showing that the efficiency of culture RNA was variable in both cases. The efficiency was higher under SO -than Mg-limited conditions.


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