Which organisms changed earth's atmosphere from reducing to oxidising?

Options

A. chemoautotrophs
B. Bacterial photautotrophs
C. Blue-green algae/oxygenic photoautotrophs
D. Chemoheterotrophs.

Answer

To answer the question "Which organisms changed Earth's atmosphere from reducing to oxidizing?", we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Earth's Atmosphere**:
- The Earth's atmosphere can be divided into two categories: the primary atmosphere (reducing) and the secondary atmosphere (oxidizing). The primary atmosphere lacked free oxygen, while the secondary atmosphere contains oxygen.

2. **Identifying the Role of Organisms**:
- The question asks which organisms were responsible for introducing oxygen into the atmosphere, thus changing it from reducing to oxidizing.

3. **Evaluating the Options**:
- **Chemoautotrophs**: These organisms obtain energy by oxidizing inorganic substances and do not produce oxygen. They were among the first to appear but did not contribute to oxygen production.
- **Bacterial Photoautotrophs**: These organisms can perform photosynthesis but often do so anaerobically, meaning they do not produce oxygen.
- **Chemoheterotrophs**: These organisms consume organic compounds for energy and do not produce oxygen.
- **Blue-Green Algae (Cyanobacteria) or Oxygenic Photoautotrophs**: These organisms perform photosynthesis using sunlight, water, and carbon dioxide, producing oxygen as a byproduct.

4. **Conclusion**:
- Among the given options, blue-green algae (also known as cyanobacteria) or oxygenic photoautotrophs are the organisms that introduced oxygen into the atmosphere, thereby changing it from reducing to oxidizing.

5. **Final Answer**:
- The correct answer is **blue-green algae or oxygenic photoautotrophs**.