Response to NaCl of nitrate assimilation and nitrogenase activity of the cyanobacterium Anabaena sp PCC 7120 and its mutants

The presence of NaCl in the nutrient solution promoted nitrate uptake in pa rent Anabaena sp. PCC 7120, mutants SP7 (defective in nitrate reductase act ivity) and SP17 (partially defective in nitrate reductase activity), but no t in the mutant SP9 (defective in nitrate transport and reduction). Nitrate reductase activity of the parent and mutant SP17 increased with increasing concentration of nitrate in saline medium, while mutants SP7 and SP9 did n ot respond to the altered salinity. Although Na+ was not required for nitra te reductase activity, its presence in the nutrient solution enhanced nitra te reduction. Complete removal of Nat from the nutrient solution markedly r educed nitrogenase activity in all the strains, while raising the concentra tion of NaCl to 50 mmol l(-1) or above, was equally toxic to nitrogenase ac tivity. External NaCl at 200 mmol l(-1) brought down the nitrogenase activi ty to the same residual level as observed without Na+.