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 Nitrosococcus.
B. Step (ii) is carried out by Nitrobacter
C. Both steps (i) and (ii) can be called nitrofication
D. bacteria carrying out thse steps are usually photoautotrophs.

Explanation

To solve the question regarding the reactions carried out by nitrogen-fixing microbes, we need to analyze the provided equations and the statements related to them. Here is a step-by-step breakdown of the solution:

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

These reactions represent the process of nitrification, where ammonia is oxidized to nitrite and then nitrite is oxidized to nitrate.

### Step 2: Identify the Bacteria Involved
- The first reaction (i) is typically carried out by bacteria such as **Nitrosomonas** or **Nitrosococcus**, which convert ammonia (NH3) to nitrite (NO2^-).
- The second reaction (ii) is carried out by **Nitrobacter**, which converts nitrite (NO2^-) to nitrate (NO3^-).

### Step 3: Analyze the Statements
We have four statements to evaluate:
1. Step one is carried out by Nitrosomonas or Nitrosococcus.
2. Step two is carried out by Nitrobacter.
3. Both step one and step two can be called nitrification.
4. Bacteria carrying out the steps are usually photoautotrophs.

### Step 4: Determine the Truthfulness of Each Statement
- Statement 1: True (Step one is indeed carried out by Nitrosomonas or Nitrosococcus).
- Statement 2: True (Step two is carried out by Nitrobacter).
- Statement 3: True (Both steps are part of the nitrification process).
- Statement 4: Not true (The bacteria involved in these reactions are **chemoautotrophs**, not photoautotrophs).

### Conclusion
The statement that is **not true** is:
**Option D: Bacteria carrying out the steps are usually photoautotrophs.**

### Final Answer
The correct answer is **Option D**.

---