Carefully read the following reactions carried out by nitrogen fixing bacteria. Identify the statement about these equations which is not true.
`3NH_(3)+3O_(2) to 2NO_(2)^(-)+2H^(+)+2H_(2)O` …………..(A)
`2NO_(2)+O_(2) to 2NO_(3)^(-)`

Options

A. Both the step (A) and (B) can be called nitrification.
B. Step (A) carried out by Nitrosomonas or Nitrococcus
C. Both the steps occur only in photoautotrophs
D. Step (B) is carried out by Nitrobacter.

Answer

To solve the question regarding the reactions carried out by nitrogen-fixing bacteria, we need to analyze the given reactions and the statements about them.

### Step-by-Step Solution:

1. **Identify the Reactions**:
- Reaction A: `3NH₃ + 3O₂ → 2NO₂⁻ + 2H⁺ + 2H₂O`
- Reaction B: `2NO₂ + O₂ → 2NO₃⁻`

2. **Understand the Process**:
- Both reactions are part of the nitrification process, which involves the conversion of ammonia (NH₃) to nitrites (NO₂⁻) and then to nitrates (NO₃⁻).
- The first step (Reaction A) is typically carried out by bacteria such as Nitrosomonas and Nitrococcus, which oxidize ammonia to nitrite.
- The second step (Reaction B) is carried out by other bacteria, converting nitrite to nitrate.

3. **Evaluate the Statements**:
- **Statement 1**: Both steps A and B can be called nitrification.
- This statement is true as both reactions are part of the nitrification process.
- **Statement 2**: Step A is carried out by Nitrosomonas and Nitrococcus.
- This statement is also true since these bacteria are known to perform the oxidation of ammonia to nitrite.
- **Statement 3**: Both the steps occur only in photoautotrophs.
- This statement is not true. Nitrifying bacteria are chemoautotrophs, meaning they obtain energy from the oxidation of inorganic substances, not from photosynthesis.

4. **Conclusion**:
- The statement that is not true is that both steps occur only in photoautotrophs. Nitrifying bacteria are not photoautotrophs; they are chemoautotrophs.

### Final Answer:
The statement that is not true is: "Both the steps occur only in photoautotrophs."

---