An organism that requires oxygen, but at levels lower than those found in the atmosphere. Some symbiotic bacteria that grow in nodules of plant roots are microaerophiles.

When grown in microaerophilic conditions, for example, the Rhizobium bacterium, found in the root nodules of certain legumes, is able to fix nitrogen, making it bioavailable to the plant host. Rhizobium needs a certain amount of oxygen to produce energy for nitrogen fixation, but too much oxygen will inactivate the nitrogenase enzyme. Oxygen levels are controlled by a protein known as leghemoglobin (probably derived from legume and hemoglobin), a red, iron containing protein that can buffer the amount of oxygen in the root nodule.

This is why soybeans are often grown in nitrogen poor soils. Because the bacteria are able to fix nitrogen, not only are the plants able to grow, but the soil nitrogen levels are replenished

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