Final answer:
Bacteria use the enzyme nitrogenase to convert nitrogen gas (N2) to ammonia (NH3) in a process known as nitrogen fixation. Nitrogen-fixing bacteria play a key role in the nitrogen cycle, and denitrifying bacteria convert nitrates back to nitrogen gas.
Step-by-step explanation:
The enzyme that bacteria use to convert nitrogen gas (N2) to ammonia (NH3) is nitrogenase. This process is known as nitrogen fixation, which is a crucial part of the nitrogen cycle. Nitrogen-fixing bacteria, often associated with the root nodules of legumes or free-living in the soil, utilize nitrogenase to convert atmospheric nitrogen into organic compounds that can be used by plants. After nitrogen is fixed, other soil bacteria play roles in converting it into different forms. Bacteria of the genus Nitrosomonas oxidize ammonia to nitrites (NO2), and Nitrobacter bacteria transform these nitrites into nitrates (NO3), which plants can absorb and use. Eventually, denitrifying bacteria like Pseudomonas and Clostridium convert nitrates back into nitrogen gas, completing the cycle.