The oxygen cycle is a global cycle of oxygen circulation between living organisms and the non-living environment. Microorganisms are an important facet of this cycle.

There is substantial evidence in the fossil record that the present atmosphere is due to the activity of bacteria, in particular to the bacteria known as cyanobacteria. Originally, the Earth's atmosphere was virtually oxygen-free. With the evolution of cyanobacteria, which derive their energy from photosynthesis with the subsequent release of oxygen, the oxygen level in the atmosphere increased. Over millions of years of bacterial (and later plant) activity, the oxygen content attained the present day level. Microorganisms such as the cyanobacteria are thus considered producers of atmospheric oxygen.

Microorganisms are also involved in the removal of oxygen from the atmosphere (i.e., they are consumers of oxygen) The process of respiration uses oxygen to produce energy. For...