The effects of low level osmotic stress on the growth and physiology of three cyanobacteria, 6301, 7122 and 29413, were investigated. No significant differences in the rates or patterns of growth were found in any strain when subjected to salinity stresses up to 100 mM-NaCl. The rates of photosynthetic oxygen evolution and nitrogenase activity were depressed rapidly and temporarily upon the addition of NaCl to the medium, to an extent which was dependent upon the amount of NaCl added. Nitrogenase activity was more sensitive to NaCl than was photosynthesis, and recovery took longer. Internal Na concentrations increased transiently upon upshock. This may be responsible for the observed inhibition of photosynthesis and nitrogenase activity, and recovery may be dependent upon the ability of the cell to adjust to sudden increases in internal Na. These results indicate that metabolic disruption is transient and that, in the long term, growth and metabolic activity remain unaffected.


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