Specialized cells for fixing atmospheric nitrogen in Nostoc are:

Heterocyst are found in many species of filamentous blue-green algae. They are cells of slightly larger size and with a more thickened wall than the vegetative cells. heterocyst are the site of nitrogen fixation in blue-green algae. The role includes: (i) fixation of nitrogen by all heterocystous algae, (ii) inhibition of heterocyst formation by combined nitrogen and (iii) direct observations on acetylene reduction by isolated heterocyst. Related Theory Heterocysts are specialized, paleyellow, thick-walled cells with the function of nitrogen-fixing formed during nitrogen starvation by some filamentous cyanobacteria, such as Nostoc and Anabaena. They fix nitrogen from dinitrogen $\left(N_2\right)$ in the air using the enzyme nitrogenase, in order to provide the cells in the filament with nitrogen for biosynthesis. Nitrogenase is inactivated by oxygen, so the heterocyst must create a micro anaerobic environment. Hence, their main function is nitrogen fixation.