The process of nitrogen cycling by microbes involves multiple steps of oxidation and simultaneous reduction. The following reaction occurring during one such step is brought about by which bacteria?
`2NO_2^- + O_2 to 2NO_3^_`

Options

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

Answer

To solve the question regarding which bacteria is responsible for the reaction `2NO_2^- + O_2 → 2NO_3^-`, we need to understand the context of the nitrogen cycle and the specific roles of different bacteria involved in this process.

### Step-by-Step Solution:

1. **Understanding the Nitrogen Cycle**:
- The nitrogen cycle is a biogeochemical cycle that describes the transformation of nitrogen and nitrogen-containing compounds in the environment. Key processes include nitrogen fixation, nitrification, assimilation, ammonification, and denitrification.

2. **Identifying the Reaction**:
- The reaction given is `2NO_2^- + O_2 → 2NO_3^-`, which indicates the conversion of nitrite (NO2^-) to nitrate (NO3^-). This process is known as nitrification.

3. **Nitrification Process**:
- Nitrification is a two-step process where ammonia is first oxidized to nitrite by bacteria such as Nitrosomonas, and then nitrite is oxidized to nitrate by another group of bacteria.

4. **Bacteria Involved in Nitrification**:
- The first step of nitrification is carried out by Nitrosomonas (which converts ammonia to nitrite), and the second step is carried out by Nitrobacter (which converts nitrite to nitrate).

5. **Conclusion**:
- Since the reaction in question specifically involves the conversion of nitrite (NO2^-) to nitrate (NO3^-), it is clear that this step is performed by Nitrobacter.

### Final Answer:
The bacteria responsible for the reaction `2NO_2^- + O_2 → 2NO_3^-` is **Nitrobacter**.