Oxygenic photosynthesis is absent in :-

Options

A. Spirulina
B. Rhodospirillum
C. Oscillatoria
D. Macrocystis

Answer

To answer the question "Oxygenic photosynthesis is absent in:", we need to analyze the given options one by one to determine which organism does not perform oxygenic photosynthesis.

### Step-by-Step Solution:

1. **Understand Oxygenic Photosynthesis**:
- Oxygenic photosynthesis is a process where organisms use water as an electron donor, and in the process, they release oxygen. This is typical of plants, algae, and cyanobacteria.

2. **Evaluate the Options**:
- **Option A: Spirulina**:
- Spirulina is a genus of cyanobacteria (blue-green algae) that performs oxygenic photosynthesis using water as an electron donor. Therefore, oxygenic photosynthesis is present in Spirulina.
- **Option B: Rhodospirillum**:
- Rhodospirillum is a genus of bacteria that performs anoxygenic photosynthesis. It uses hydrogen sulfide instead of water as an electron donor, meaning it does not perform oxygenic photosynthesis. Thus, oxygenic photosynthesis is absent in Rhodospirillum.
- **Option C: Oscillatoria**:
- Oscillatoria is also a genus of cyanobacteria and performs oxygenic photosynthesis, similar to Spirulina. Therefore, oxygenic photosynthesis is present in Oscillatoria.
- **Option D: Macrocystis**:
- Macrocystis is a genus of brown algae (eukaryotic organisms) that also performs oxygenic photosynthesis. Thus, oxygenic photosynthesis is present in Macrocystis.

3. **Conclusion**:
- Based on the analysis, the only organism from the options that does not perform oxygenic photosynthesis is **Rhodospirillum**. Therefore, the correct answer is **Option B**.

### Summary:
- **Correct Answer**: Option B: Rhodospirillum