Cyanobacteria are characterised by

Options

A. ability to do oxygenic photosynthesis and absence of phycobilins
B. ability to perform oxygenic photosynthesis and presence of phycobilins
C. presence of chlorophyll in chloroplast
D. ability to do anoxygenic photosynthesis and presence of nitrogenase

Answer

### Step-by-Step Solution:

1. **Understanding Cyanobacteria**:
- Cyanobacteria, also known as blue-green algae, are prokaryotic organisms. This means they lack a true nucleus and membrane-bound organelles. They can exist as unicellular organisms or in colonies.

2. **Habitat**:
- They are predominantly found in aquatic environments, including freshwater and marine ecosystems, although they can also be found in terrestrial habitats.

3. **Photosynthesis**:
- Cyanobacteria are capable of photosynthesis, which allows them to produce their own food. They utilize photosynthetic pigments to capture light energy.

4. **Oxygenic Photosynthesis**:
- During photosynthesis, cyanobacteria release molecular oxygen (O₂) as a byproduct. This process is referred to as oxygenic photosynthesis.

5. **Pigments**:
- They contain various pigments, including chlorophyll, which is essential for photosynthesis. Additionally, they have phycobilins, which are blue pigments that contribute to their coloration and are found in structures called phycobilisomes.

6. **Analyzing the Options**:
- **Option A**: Ability to do oxygenic photosynthesis and absence of phycobilins - Incorrect, as cyanobacteria have phycobilins.
- **Option B**: Ability to perform oxygenic photosynthesis and presence of phycobilins - Correct, as both statements are true.
- **Option C**: Presence of chlorophyll in chloroplast - Incorrect, as cyanobacteria do not have chloroplasts; they have chlorophyll in their thylakoid membranes.
- **Option D**: Ability to do anoxygenic photosynthesis and presence of nitrogenase - Incorrect, as cyanobacteria perform oxygenic photosynthesis, not anoxygenic.

7. **Final Answer**:
- The correct option is **B**: Ability to perform oxygenic photosynthesis and presence of phycobilins.