Question

Primary function of heterocyst in Nostoc is
A) Food storage
B) Nitrogen fixation
C) Reproduction
D) Protection

Answer 1

Hint: Nostoc is a cyanobacterium. It has the ability to fix atmospheric nitrogen. It converts nitrogen into ammonia. The heterocysts present in it are specialized for nitrogen fixation. It contains the enzyme nitrogenase. Nitrogenase is inactivated by oxygen. Hence, heterocyst creates a micro anaerobic environment.

Complete Answer:
Heterocysts are found in numerous species of the filamentous blue‐green algae. They are cells of slightly larger size and with a more thickened wall than the vegetative cells. They fix nitrogen from di-nitrogen ($N^2$) in the air using the enzyme nitrogenase, to provide the cells in the filament with nitrogen for biosynthesis.
Nitrogenase is deactivated by oxygen, so the heterocyst creates a micro anaerobic environment. Hence, their main function is nitrogen fixation.
Nostoc is a cyanobacterium. It can fix atmospheric nitrogen. It converts nitrogen into ammonia.
The heterocysts present in it are specialized for nitrogen fixation. It contains the enzyme nitrogenase. Nitrogenase is inactivated by oxygen. Hence, heterocyst creates a micro anaerobic environment.
There are two types of nitrogen-fixing bacteria one is free-living and another is symbiotic. Some bacteria and blue-green algae like Nostoc are free-living bacteria and able to fix nitrogen from the atmosphere to enrich the soil with nitrogen and increase soil fertility. Rhizobium is the symbiotic bacteria that live in root nodules of leguminous plants and fixes the atmospheric nitrogen in plants in the usable form.

Thus, the correct answer is option B.

Note: Heterocyst does not possess PS II and hence cannot produce oxygen.