Oxygen cycle in microorganisms

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 example, the decay of organic material by microorganisms such as bacteria and fungi consumes oxygen. The microbial decomposition process involves numerous species of bacteria and fungi. Some of these release oxygen.

Microorganisms also contribute to the oxygen cycle in an indirect way. For example, the degradation of organic compounds (e.g., cellulose) by bacteria can make the compounds capable of being used as a food source by another organism. This subsequent utilization can both consume and produce oxygen at various stages of the digestive process.

The oxygen cycle in microorganisms also operates at a much smaller scale. The best example of this is the stratification of microbial life in water that occurs due to the oxygen concentration. Oxygen does not dissolve easily in water. Thus, oxygen from the atmosphere enters water very slowly. In a body of water—for example, a lake—the result is a higher concentration of oxygen in the uppermost region of the water. Those bacteria that produce oxygen (i.e., cyanobacteria) will also be located in this surface region of the water, because Sunlight is most available there. Food sources that are not consumed by these bacteria and other surface-dwelling life sink to deeper water. In the deeper water, bacteria and other microorganisms that can live in the presence of low oxygen levels then utilize the nutrients. At the greatest depths reside microorganisms that cannot tolerate oxygen. These anaerobic microorganisms degrade the nutrients that reach the bottom. This stratification of microbial life will affect the presence of other life in the water, as well as the cycling of other compounds (e.g., the carbon cycle).

The oxygen cycle in microorganisms in bodies of water such as lakes and rivers can have important consequences on the health of the water. For example, if mixing of the water in the lake does not readily occur, the body of water can become stagnant. In other words, the oxygen content of the water becomes depleted and, without mixing, insufficient surface level oxygen is available to replenish the supply. Fish life in the water can suffer. Another example is the depletion of oxygen from a lake by the explosive growth of algae. The algal "bloom" can essentially make the water body incapable of supporting life. Furthermore, if the algal species is a toxin producer, the water can become hazardous to health. A final example is the relationship between the oxygen cycle in microorganisms and water pollution . Polluted water is typically enriched in nutrients that will support the rapid growth of bacteria and other microbes. Their growth depletes the oxygen in the water. In grossly polluted water, this depletion can be so extensive that the water cannot support oxygen-dependent life.

Thus, the oxygen cycle in microorganisms, mainly bacteria, is very important in determining the quality of a water body and so of the ability of the water to be a productive source of life.

See also Carbon cycle in microorganisms; Composting, microbiological aspects; Life, origin of; Nitrogen cycle in microorganisms