Magnesium-limited Growth of the Cyanobacterium Anacystis nidulans

Botanical Laboratory, University of Oslo, PO Box 1045, Blindern, Oslo 3, Norway ABSTRACT SUMMARY: Mg 2+ -limited growth of the cyanobacterium Anacystis nidulans was investigated in batch and chemostat cultures. In batch cultures the growth rate of the organism depended on the Mg 2+ concentration up to 5 µ M . 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 2+ concentration from 5 µ M to 1 m 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 2+ shift-up experiments, showing that the control by Mg 2+ of growth had been separated from its control of cell division, In Mg 2+ -limited chemostat cultures, the mean cell volume decreased from about 2.0 to 0.6 µm 3 when the Mg 2+ concentration was increased from 2.5 to 10 µ M . This increase in Mg 2+ also resulted in an increase in the calculated intracellular Mg 2- concentration from 27 to 78 m M , and the amount of cellular Mg 2+ bound in chlorophyll increased from 17 to 22%. A comparison of Mg 2+ - and SO 4 2- -limited chemostat cultures showed that the mean cell volume decreased with increasing dilution rate when Mg 2+ was the limiting factor, whereas it increased with dilution rate when SO 4 2- 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 2+ -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 4 2- - and Mg 2+ -limited chemostat cultures showing that the efficiency of culture RNA was variable in both cases. The efficiency was higher under SO 4 2- -than Mg 2+ -limited conditions.