ENVIRONMENTAL-REGULATION OF NITROGENASE AND NITRATE REDUCTASE AS SYSTEMS OF NITROGEN ASSIMILATION IN THE AZOLLA-MEXICANA-ANABAENA-AZOLLAE SYMBIOSIS

The effect of various environmental factors on the activities of nitrate reductase and nitrogenase of the Azolla mexicana-Anabaena azollae symbiosis were determined. Nitrate reductase activity was maximally induced by 5 mM nitrate nitrogen and unaffected by either short-term influences of ammonium-nitrogen (10 mM), oxygen (100%), or varying pH (4-8). Temperature optimum for activity was 30°C. Its activity was also inhibited 50% by -10 bars of osmotic stress (polyethylene glycol - 6000). Nitrogen fixation, found only in the phycobiont, was also unaffected by pH (4-8). The nitrogen fixation activity was optimal between 20 and 30°C; osmotic stress reduced the activity 50% at -2.6 bars, nitrate-nitrogen at levels less than 10 mM reduced it 50%, while ammonium-nitrogen (10mM) reduced it only 32%. Oxygen at levels above natural concentration (21%) reduced nitrogen fixation activity. Below 21% O2, fixation in the symbiosis was unaltered while it increased in the isolated alga with decreasing pO2. © 1979.