Regulation of nitrogenase activity by light in the azolla-anabaena symbiosis

Nitrogenase activity and the rate of photosynthesis were measured simultaneously in Azolla by a continuous gas flow system. The mode of interaction between light, photosynthesis and nitrogenase activity was analysed. Nitrogenase activity dropped off when either Azolla plants or the cyanobiont Anabaena were transferred from light to dark. This decline was immediate and was independent of length or intensity of the prior light phase. Reillumination restored nitrogenase activity. Nitrogenase activity did not depend on the rate of photosynthesis at light intensities below 10 μE m^-2 s^-1. Its activity was saturated at 200 μE m^-2 s^-1 while CO₂ fixation was saturated at a light intensity of 850 μE m^-2 s^-1. Azolla photosynthetic activity followed the absorption spectrum of chlorophyll a, while nitrogenase activity markedly increased between 690 and 710 nm. Inhibition of photosynthesis by DCMU was accompanied by an increase in nitrogenase activity. These results suggest direct light regulation of nitrogenase activity in Azolla independent of CO₂ fixation, and a possible inhibition of nitrogenase activity by the oxygen produced in photosynthesis.