**Specialized cells for fixing atmospheric nitrogen in nostoc are A. Akinetes B. Heterocysts C. Hormogonia D. Nodules**

**B. Heterocysts:**
Heterocysts are specialized cells found in the filamentous cyanobacterium Nostoc that are responsible for fixing atmospheric nitrogen. These cells play a crucial role in the nitrogen cycle by converting atmospheric nitrogen gas (N2) into a form that can be utilized by other organisms.

Heterocysts are larger and more rounded than the other cells in the filament. They have thick cell walls and lack photosynthetic pigments, making them appear pale or colorless. This lack of pigments is essential to create an anaerobic environment within the heterocyst, as nitrogenase, the enzyme responsible for nitrogen fixation, is sensitive to oxygen.

**Process of Nitrogen Fixation:**
1. Nitrogen fixation occurs in the heterocysts through a series of enzymatic reactions.
2. Oxygen is actively excluded from the heterocyst by the thick cell wall and the presence of specialized proteins called oxygen-scavenging proteins.
3. The enzyme nitrogenase found in the heterocyst catalyzes the conversion of atmospheric nitrogen (N2) into ammonia (NH3), a form that can be assimilated by other cells.
4. The ammonia produced in the heterocyst is then transported to neighboring cells through gap junctions called microplasmodesmata.
5. These neighboring cells, called vegetative cells, provide the heterocyst with photosynthetically-derived carbohydrates as an energy source in exchange for the fixed nitrogen.

**Significance of Heterocysts:**
1. Heterocysts are crucial in sustaining the growth and survival of Nostoc in nitrogen-deficient environments.
2. They enable the conversion of atmospheric nitrogen into a usable form, which is essential for the synthesis of proteins, nucleic acids, and other nitrogen-containing compounds.
3. The fixed nitrogen produced by heterocysts can be utilized by other organisms in the ecosystem, contributing to the overall nitrogen cycle.
4. Heterocysts also play a role in maintaining the balance of oxygen within the filamentous cyanobacterium, as excessive oxygen can inhibit nitrogen fixation.

In conclusion, heterocysts are specialized cells in Nostoc that are responsible for fixing atmospheric nitrogen. These cells create an anaerobic environment, allowing the enzyme nitrogenase to convert atmospheric nitrogen into ammonia. The fixed nitrogen is then transferred to neighboring cells for the synthesis of essential compounds. Heterocysts are vital to the nitrogen cycle and the survival of Nostoc in nitrogen-deficient environments.

These are called "Heterocysts"