A nitrite oxidising bacterium is ...........

Explanation

### Step-by-Step Solution:

1. **Understanding Nitrite Oxidizing Bacteria**:
- Nitrite oxidizing bacteria (NOB) are a group of bacteria involved in the nitrification process, specifically in the second step where nitrite (NO2-) is converted into nitrate (NO3-).

2. **Nitrification Process**:
- Nitrification is a two-step process:
- The first step involves the oxidation of ammonia (NH3) to nitrite (NO2-) by ammonia-oxidizing bacteria (AOB).
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