Choose the incorrect statement about cyanobacteria.

Options

A. They can fix N,
B. Protoplasm is divided into centroplasm and chromoplasm
C. Sexual reproduction occurs by conjugation
D. Spiral Gram positive Gram negative nature of cell wall

Answer

### Step-by-Step Solution:

1. **Understand the Characteristics of Cyanobacteria**:
- Cyanobacteria, also known as Blue Green Algae, are free-living prokaryotic organisms.
- They are capable of nitrogen fixation, which is an important ecological role.
- Their protoplasm is divided into two parts: centroplasm and chromoplasm. The chromoplasm contains thylakoids, which are involved in photosynthesis.

2. **Reproductive Methods**:
- Cyanobacteria do not have sexual organs or gametes due to their prokaryotic nature.
- They reproduce through vegetative and asexual methods.
- There is a process called parasexual reproduction, which involves genetic recombination through transformation and conjugation.

3. **Cell Wall Structure**:
- Cyanobacteria have a thick layer of peptidoglycan in their cell wall, which can make them gram-positive or gram-negative.

4. **Evaluate the Statements**:
- **Statement 1**: Cyanobacteria can fix nitrogen. (Correct)
- **Statement 2**: Protoplasm is divided into centroplasm and chromoplasm. (Correct)
- **Statement 3**: Sexual reproduction occurs by conjugation. (Incorrect)
- **Statement 4**: They possess both gram-positive and gram-negative characteristics due to their thick peptidoglycan layer. (Correct)

5. **Identify the Incorrect Statement**:
- The only incorrect statement is that sexual reproduction occurs by conjugation, as cyanobacteria do not reproduce sexually.

6. **Conclusion**:
- Therefore, the incorrect statement about cyanobacteria is that sexual reproduction occurs by conjugation.

### Final Answer:
**Option 3: Sexual reproduction occurs by conjugation.**

---