Abstract:The nitrogenase activity ( by acetylene reduction ) was measured in the blue-green algae Anabaena 7120 upon transition from preincubation in dark to light condition.The following results were obtained.On transferrig Anabaena preincubated from dark to light condition, there was no nitrogenase activity at the begining of incubation, but the activity was expressed just after 2-4 h illumination and was increased with illuminating time. The addition of 0.5% sucrose to the test systems had an enhancing effect on the nitrogenase activity expression in Anabaena preincubated in dark.When algae cells were preincubated in dark under an atmosphere containing 100% Ar, or 15% O2,or 100% N2, or in presence of photosynthetic inhibitors such as DNP or CCCP, and MSX, the depression of nitrogenase activity of Anabaena was observed. But the enhancing effect was found when inhibitors, O2, N2 and MSX were added stimultaneously with sucrose.The nitrogenase activity in Anabaena preincubated in dark was promoted, but was decreased in Anabaena preincubated in light under molecular hydrogen.The addition of H2 together with O2 and sucrose could enhance nitrogenase activity after being depressed by the factors mentioned above.