Oxygenic photosynthesis is absent in :-

Options

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

Answer

### Step-by-Step Solution:

1. **Understanding Photosynthesis Types**:
- Photosynthesis can be categorized into two main types: oxygenic and anoxygenic.
- Oxygenic photosynthesis involves the use of water (H2O) as an electron donor, producing oxygen (O2) as a byproduct.
- Anoxygenic photosynthesis uses substances like hydrogen sulfide (H2S) as an electron donor and does not produce oxygen.

2. **Identifying the Organisms**:
- The question lists four organisms: Spirulina, Rhodospirilium, Oscillatoria, and Macrocystis.
- We need to determine which of these organisms does not perform oxygenic photosynthesis.

3. **Analyzing Each Organism**:
- **Spirulina**: This is a genus of cyanobacteria that performs oxygenic photosynthesis. It uses water as an electron donor.
- **Rhodospirilium**: This is a type of purple bacteria that performs anoxygenic photosynthesis, using H2S instead of water. Therefore, it does not produce oxygen.
- **Oscillatoria**: Another genus of cyanobacteria that also performs oxygenic photosynthesis, using water as an electron donor.
- **Macrocystis**: This is a genus of brown algae that also performs oxygenic photosynthesis, using water.

4. **Conclusion**:
- Among the listed organisms, **Rhodospirilium** is the only one that does not perform oxygenic photosynthesis. Thus, the correct answer is **Rhodospirilium**.

### Final Answer:
Oxygenic photosynthesis is absent in **Rhodospirilium**.