NO3- nutrition and salt tolerance in the cyanobacterium Anabaena sp. PCC 7120 and mutant strains.

Growth of the cyanobacterium Anabaena sp. PCC 7120 and its nitrate assimilation-defective mutants was inversely proportional to the NaCl concentration in the medium. Presence of nitrate in the saline medium protected the growth of the parent but not of the mutant strains from salt toxicity. On the other hand, ammonium nitrogen protected the growth of all the strains from salt toxicity. However, the effect was less than that of nitrate. An altered sodium transport system was evident in the mutant strains and was most marked in mutant SP9. The cellular sodium concentration in parent and mutant strains also varied. Although mutant SP9 exhibited the lowest level of cellular sodium, it was as sensitive to salt toxicity as other strains. It is assumed that merely the presence of a toxic level of NaCl in the ambient environment is sufficient to damage the structural and functional components of the plasma membrane.