The first organisms which released oxygen in the environment by its photosynthesis were :

Options

A. Cyanobacteria
B. Sulpher bacteria
C. Iron bacteria
D. Planktonic sulpher bacteria

Answer

To solve the question regarding the first organisms that released oxygen into the environment through photosynthesis, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Context**:
- The question asks about the first organisms that released oxygen through photosynthesis. We need to consider the timeline of the evolution of photosynthetic organisms.

2. **Identifying the Type of Organisms**:
- The earliest life forms were prokaryotic organisms. Among these, the first photosynthetic organisms were likely anaerobic bacteria, as they evolved in an environment that lacked free oxygen.

3. **Evolution of Photosynthetic Bacteria**:
- Anaerobic bacteria were the first to evolve, as they could survive without oxygen. However, they did not produce oxygen as a byproduct of photosynthesis.

4. **Transition to Aerobic Photosynthesis**:
- Approximately 3.2 billion years ago, aerobic bacteria evolved. These organisms utilized water (H2O) in their photosynthesis process, which resulted in the release of oxygen (O2) into the atmosphere.

5. **Identifying the Specific Organism**:
- The first aerobic photosynthetic bacteria are believed to be cyanobacteria, also known as blue-green algae. They are significant because they are responsible for the oxygenation of the atmosphere.

6. **Fossil Evidence**:
- The oldest known fossils of these organisms are called *Archaeospheroids barbatonensis*, which supports the claim that cyanobacteria were among the first to release oxygen.

7. **Conclusion**:
- Based on the information gathered, the correct answer to the question is that the first organisms to release oxygen into the environment through photosynthesis were cyanobacteria.

### Final Answer:
**Cyanobacteria (blue-green algae)**

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