Select the CORRECT statement/s regarding heterocysts.

Specialized nitrogen-fixing cells found in some filamentous cyanobacteria are called heterocyst. These are modified to fix atmospheric nitrogen when faced with nitrogen starvation. Heterocyst contains the enzyme nitrogenase which helps in fixing N₂ and provides nitrogen for cell biosynthesis. Since nitrogenase gets deactivated in the presence of oxygen, the heterocyst creates a micro-anaerobic environment. These cells undergo certain physiological changes like the presence of three additional cell walls, degradation of PS-II to facilitate the fixing process. Anabaena, Nostoc, etc. are common examples of having heterocyst.