Nitrate formation by the following reaction is accomplished by
`2NO_2^(-)+O_2rarr2NO_(3)^-`

Options

A. Nitrococcus
B. Nitrosomonas
C. Nitrobacter
D. Thiobacillus or Pseudomonas

Answer

To solve the question regarding the nitrate formation by the given reaction, we need to analyze the process of nitrification and identify the bacteria involved in the specific step of nitrite oxidation to nitrate.

### Step-by-Step Solution:

1. **Understanding the Reaction**:
The reaction provided is:
\[ 2NO_2^{-} + O_2 \rightarrow 2NO_3^{-} \]
This indicates that nitrite ions (NO₂⁻) are being oxidized to nitrate ions (NO₃⁻) in the presence of oxygen (O₂).

2. **Identifying the Process**:
This reaction is a part of the nitrification process, which is the biological conversion of ammonia into nitrate. Nitrification occurs in two main steps:
- **Step 1**: Ammonia (NH₃) is oxidized to nitrite (NO₂⁻) by specific bacteria.
- **Step 2**: Nitrite (NO₂⁻) is further oxidized to nitrate (NO₃⁻) by other bacteria.

3. **Bacteria Involved**:
- In the first step of nitrification, the bacteria responsible for converting ammonia to nitrite are **Nitrosomonas** and **Nitrococcus**.
- In the second step, where nitrite is oxidized to nitrate, the bacteria responsible for this conversion are **Nitrobacter**.

4. **Conclusion**:
Therefore, the bacteria responsible for the nitrate formation from nitrite in the given reaction is **Nitrobacter**.

### Final Answer:
The nitrate formation by the given reaction is accomplished by **Nitrobacter**.