Microbiology and Immunology

Oxygen cycle in microorganisms WORLD OF MICROBIOLOGY AND IMMUNOLOGY

418

•

OOxygen cycle in microorganismsXYGEN CYCLE IN MICROORGANISMS

The oxygen cycle is a global cycle of oxygen circulation

between living organisms and the non-living environment.

Microorganismsare 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 partic-

ular to the bacteria known as cyanobacteria. Originally, the

Earth’s atmosphere was virtually oxygen-free. With the evolu-

tionof cyanobacteria, which derive their energy from photo-

synthesiswith 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 cyanobac-

teria 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 oxy-

gen) The process of respiration uses oxygen to produce

energy. For example, the decay of organic material by

microorganisms such as bacteria and fungiconsumes 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 com-

pounds (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 stratifica-

tion 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 con-

sumed by these bacteria and other surface-dwelling life sink to

deeper water. In the deeper water, bacteria and other microor-

ganisms that can live in the presence of low oxygen levels then

utilize the nutrients. At the greatest depths reside microorgan-

isms that cannot tolerate oxygen. These anaerobic microor-

ganisms 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

Life on Earth, made possible because of oxygen.

womi_O 5/7/03 7:58 AM Page 418