Heterocysts are specialised in

Options

A. Nitrogen fixation under aerobic conditions
B. Nitrogen fixation under anaerobic conditions
C. Formation of internal hormogones
D. Saprotrophism

Answer

### Step-by-Step Solution:

1. **Understanding Heterocysts**: Heterocysts are specialized cells found in certain cyanobacteria, which are also known as blue-green algae.

2. **Function of Heterocysts**: The primary function of heterocysts is to facilitate nitrogen fixation. This process is crucial for converting atmospheric nitrogen (N₂) into a form that can be utilized by living organisms.

3. **Presence of Nitrogenase**: Heterocysts contain the enzyme nitrogenase, which is essential for the nitrogen fixation process. However, nitrogenase is sensitive to oxygen.

4. **Anaerobic Conditions**: Heterocysts create a micro-anaerobic environment that protects nitrogenase from oxygen. This allows the enzyme to function effectively and carry out nitrogen fixation.

5. **Conclusion**: Therefore, the correct answer to the question "Heterocysts are specialised in" is nitrogen fixation.

### Final Answer:
Heterocysts are specialized in nitrogen fixation.