The enzyme nitrogenase is inhibited by oxygen. In cyanobacteria, nitrogenase activity is protected by the following mechanism:

Question

Quizplus · Accepted Answer

Option A is true as heterocysts in filamentous cyanobacteria, such as Anabaena, protect nitrogenase from oxygen by means of a thickened cell wall and the absence of oxygen-generating photosynthesis. 
Option B is true as Azotobacter species produce nitrogenase-protecting proteins to counteract the inhibition of the enzyme by oxygen. 
Option C is true as cyanobacteria perform nitrogen fixation during the night when photosynthesis is not occurring, spatially separating the two processes.

The Enzyme Nitrogenase Is Inhibited by Oxygen