Question: what are heterocysts in nostoc?
Answer: Heterocysts or heterocytes are specialized nitrogen-fixing cells formed during nitrogen starvation by some filamentous cyanobacteria, such as Nostoc punctiforme, Cylindrospermum stagnale, and Anabaena sphaerica. ... Nitrogenase is inactivated by oxygen, so the heterocyst must create a microanaerobic environment.