Heterocyst Characteristics: Analysis

Heterocysts are specialized cells formed by some filamentous cyanobacteria for nitrogen fixation. Let's analyze the given characteristics:

Analysis of Statements

• (I) Thick cell wall: Heterocysts possess a thickened cell wall, including a special outer layer called the cyanophycin layer and potentially an additional outer envelope (oucide). This structure helps protect the oxygen-sensitive nitrogenase enzyme. Thus, statement (I) is correct.
• (II) Only PSI activity: Vegetative cells have both Photosystem I (PSI) and Photosystem II (PSII). Heterocysts differentiate to fix nitrogen, a process requiring an anaerobic environment. They lose PSII activity, which produces oxygen, but retain PSI activity for ATP production needed for nitrogen fixation. Thus, statement (II) is correct.
• (III) Nitrogenase activity: The primary function of heterocysts is nitrogen fixation, carried out by the enzyme nitrogenase. This enzyme is crucial for converting atmospheric nitrogen ($N_2$) into ammonia ($NH_3$). Thus, statement (III) is correct.

Conclusion

Since all three statements (I), (II), and (III) accurately describe the characteristics of heterocysts, the correct option includes all of them.