Zinc tolerance in strains of the blue-green algaAnacystis nidulans

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Abstract
Mutants of Anacystis nidulans tolerant to high levels of zinc were obtained by repeated subculturing at concentrations of the metal sufficient to cause a marked reduction in growth rate. The level of Zn just permitting detectable growth was raised from 1·45 to 16·5 mg Zn 1-1 after 75 subcultures. Two strains tolerating elevated levels of Zn were chosen for comparison with the wild-type. Zinc toxicity to all three strains was reduced by increases in Ca, Mn and P, but the tolerant strains differed from the wild-type in their response to four other variables, Na, Mg, Fe III and pH. In the case of pH, the responses of wild-type and tolerant strains were contrasting: a rise in pH from 6·5 to 8·0 led to decreased Zn toxicity to the former and increased toxicity to the latter. Increased levels of zinc led to increased formation of filaments by all strains. Although the tolerant strains required more zinc to bring about an equivalent toxic and morphological response, there was no evidence from uptake experiments that the tolerant strains excluded zinc. Uptake was measured throughout growth in batch culture and estimated as the Zn remaining after cells were washed with ethylenediaminetetra-acetic aid.