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 `2NO2^- + O2 → 2NO3^-`, we can break down the process step by step:

### Step 1: Understand the Reaction
The given reaction involves the conversion of nitrite ions (NO2^-) into nitrate ions (NO3^-). This process is part of the nitrogen cycle, specifically the nitrification process.

**Hint:** Identify the reactants and products in the reaction to understand what is being transformed.

### Step 2: Identify the Process
The conversion of nitrite (NO2^-) to nitrate (NO3^-) is a specific step in the nitrification process. Nitrification is a two-step process where ammonia (NH4^+) is first converted to nitrite by one type of bacteria, and then nitrite is converted to nitrate by another type of bacteria.

**Hint:** Remember that nitrification involves two distinct steps and different bacteria.

### Step 3: Determine the Bacteria Involved
1. **Nitrosomonas**: This bacterium is responsible for the first step of nitrification, where ammonia (NH4^+) is converted into nitrite (NO2^-).
2. **Nitrobacter**: This bacterium is responsible for the second step of nitrification, where nitrite (NO2^-) is converted into nitrate (NO3^-).

**Hint:** Recall which bacteria are associated with each step of the nitrification process.

### Step 4: Conclusion
Since the reaction given in the question specifically involves the conversion of nitrite (NO2^-) to nitrate (NO3^-), the bacterium responsible for this reaction is **Nitrobacter**.

**Final Answer:** The bacteria responsible for the reaction `2NO2^- + O2 → 2NO3^-` is **Nitrobacter**.