Why must a legume root nodule stay free of oxygen?

1. Soil bacteria collectively called rhizobia interact with legumes to form nodules on the roots. 2. The nodule is the specialized structure where nitrogen fixation happens. 3. Nitrogenase is the enzyme that fixes nitrogen, and oxygen inactivates it. 4. The nodule therefore supplies an oxygen-free area so the enzyme can work. 5. The plant gains usable ammonia while the bacteria gain photosynthates and a protected niche. 6. Photosynthesis happens in the leaves, so its absence underground is normal and explains nothing here. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 22 "Prokaryotes: Bacteria and Archaea", section 22.5 Beneficial Prokaryotes_