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