SUMMARY: Two blue-green algae from marine environments, , vigorously fixed N-labelled nitrogen in artificial sea-water free from combined nitrogen and, rather faster, in natural sea-water. A proportion of the nitrogen fixed was liberated into the medium. After a 7-day exposure period the extracellular nitrogen had a lower enrichment with N than the intracellular nitrogen. Fixation rates were not markedly affected by large variations in salinity of the medium. Calothrix was more resistant to changes in salinity than was Nostoc. Fixation in sea-water collected at various seasons was highest in January samples and lowest in July samples; additions of phosphate and of trace elements to the latter stimulated fixation. Ammonium-nitrogen did not inhibit fixation completely. It is probable that these species fix some nitrogen in Nature.


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