The extracellular products ofAnabaena cylindricaLemm. III. Excretion and uptake of fixed nitrogen

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Abstract
When Anabaena cylindrica Lemm. was resuspended in fresh medium nearly 7% of the cellular fixed nitrogen was excreted in the first 2 h. According to 15N-labelling patterns, only 2% of the combined nitrogen that appeared in the extracellular fraction during the first half-hour had been fixed during that period. In longer term experiments it was found that fixed nitrogen which had spent in excess of 9 days in the cells was excreted. This indicates a slow rate of turnover of nitrogen compounds in the cells and a considerable delay between the processes of nitrogen fixation and subsequent excretion. The possibility that some freshly fixed substances are excreted and then rapidly reabsorbed is not precluded but cannot be readily demonstrated by the 15N technique used here. When the alga was grown in the filtrate of a culture maintained for 12 days under an atmosphere of 15N-labelled N2, only 4–5% of the labelled extracellular nitrogen was taken up. However, the fungus Penicillium frequentans, isolated from a contaminated culture of the alga, assimilated nearly 19% of the extracellular nitrogen.