Name the earliest oxygenic photosynthesisers

Answer

### Step-by-Step Solution:

1. **Understanding Oxygenic Photosynthesis**:
- Oxygenic photosynthesis is a process where organisms convert light energy into chemical energy, producing oxygen as a byproduct. This process involves the intake of carbon dioxide and the release of oxygen.

2. **Identifying the Organisms**:
- The question asks for the earliest organisms that perform this type of photosynthesis. To answer this, we need to consider the history of photosynthetic organisms.

3. **Exploring Kingdom Monera**:
- Kingdom Monera is known to contain the most primitive forms of life on Earth. It includes prokaryotic organisms, which are simple and ancient.

4. **Focusing on Cyanobacteria**:
- Within Kingdom Monera, the group known as cyanobacteria (also referred to as blue-green algae) is recognized for its ability to perform oxygenic photosynthesis. These organisms are believed to be among the first to produce oxygen through photosynthesis.

5. **Conclusion**:
- Therefore, the earliest oxygenic photosynthesizers are cyanobacteria.

### Final Answer:
The earliest oxygenic photosynthesizers are cyanobacteria.

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