Reaction carried out by `N_(2)` fixing microbes include
`2NH_(3)+3O_(2)to2NO_(2)^(-)+2H^(+)+2H_(2)O` …(i)
`2NO_(2)^(-)+O_(2)to2NO_(3)^(-)` ….(ii) Which of the following statements about these equations is not true?

Options

A. step (i) is carried out by Nitrosomonas or Nitrococcus
B. step (ii) is carried out by Nitrobacter
C. Both steps (i) and (ii) can be called nitrification
D. Bacteria carrying out these steps are usually photoautotrophs

Explanation

To solve the question regarding the reactions carried out by nitrogen-fixing microbes, we will analyze the provided equations and the associated statements.

### Step-by-Step Solution:

1. **Identify the Reactions**:
The two reactions given are:
- Reaction (i): \( 2NH_3 + 3O_2 \rightarrow 2NO_2^- + 2H^+ + 2H_2O \)
- Reaction (ii): \( 2NO_2^- + O_2 \rightarrow 2NO_3^- \)

2. **Determine the Microbes Involved**:
- The first reaction (i) is typically carried out by bacteria such as **Nitrosomonas** or **Nitrococcus**. These bacteria oxidize ammonia (NH3) to nitrite (NO2^-).
- The second reaction (ii) is carried out by **Nitrobacter**, which oxidizes nitrite (NO2^-) to nitrate (NO3^-).

3. **Define the Process**:
- Both reactions are part of a process known as **nitrification**, which is the conversion of ammonia into nitrate through intermediate nitrite.

4. **Evaluate the Statements**:
- The statements regarding the reactions include:
1. The first step is carried out by Nitrosomonas or Nitrococcus.
2. The second step is carried out by Nitrobacter.
3. Both steps can be called nitrification.
4. The bacteria carrying out these steps are usually **photoautotrophs**.

5. **Identify the Incorrect Statement**:
- The fourth statement is not true. The bacteria involved in these reactions (Nitrosomonas, Nitrococcus, and Nitrobacter) are **chemoautotrophs**, not photoautotrophs. Chemoautotrophs obtain energy by oxidizing inorganic substances, while photoautotrophs use light energy.

### Conclusion:
The statement that is not true is: "The bacteria carrying out these steps are usually photoautotrophs."

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