The atmosphere became oxidising with the appearance of

Options

A. Autotrophic bacteria
B. Autotrophic cyanobacteria
C. chemoautotropic
D. chemoheterotropic

Answer

**Step-by-Step Solution:**

1. **Understanding the Question:**
The question asks which organisms contributed to the atmosphere becoming oxidizing. This refers to the introduction of free oxygen into the atmosphere.

2. **Historical Context:**
- The Earth was formed approximately 4.6 billion years ago.
- The first life forms appeared around 3.5 billion years ago.
- Initially, the atmosphere was reducing, meaning it lacked free oxygen.

3. **Identifying the Organisms:**
- The options provided are:
1. Autotropic bacteria
2. Autotropic cyanobacteria
3. Chemotropic
4. Chemoheterotropic

4. **Analyzing Each Option:**
- **Autotropic Bacteria:** These bacteria typically respire anaerobically (without oxygen). Therefore, they do not contribute to an oxidizing atmosphere. **(Incorrect)**
- **Autotropic Cyanobacteria:** These organisms are capable of photosynthesis, which produces oxygen as a byproduct. They require oxygen for respiration and contribute to an oxidizing atmosphere. **(Correct)**
- **Chemotropic:** These organisms obtain energy from chemical reactions and do not produce oxygen. They are also anaerobic. **(Incorrect)**
- **Chemoheterotropic:** These organisms rely on organic compounds for energy and do not produce oxygen. They are also anaerobic. **(Incorrect)**

5. **Conclusion:**
The correct answer is **Autotropic Cyanobacteria**, as they are responsible for the production of oxygen through photosynthesis, leading to an oxidizing atmosphere.

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