Oxygen CyclingOxygen (O) atoms cycle through the ecosystem and the biosphere the way other elements do (especially carbon). The Earth has a fixed supply of the element even though it can be found everywhere, including the atmosphere, the oceans, rocks, and all living organisms. While not all organisms need to breathe oxygen, there is definitely oxygen inside of every organism.
Oxygen Rarely AloneOxygen is one of the major compounds found in the atmosphere of the Earth. You never find oxygen floating around as individual atoms. Oxygen is always with other elements. You may find an oxygen molecule that has two oxygen atoms. There are molecules with three oxygen atoms called ozone. You will also find oxygen bound in water molecules and carbon dioxide. That oxygen floats through the atmosphere until it comes down to Earth and starts one of many cycles.
Dissolved In WaterThere is a large amount of oxygen dissolved in the water of oceans, lakes, and streams. As water moves, the oxygen is forced into solution. The organisms that live in the water breathe that oxygen by filtering it out of solution the way we do with the air. Over millions of years oxygen has also become an integral element in our rocks and land. Oxygen bonds with silicon (silicates), iron, and carbon (carbonates) to form many of the compounds in rock. Creatures like lichen are able to break down the rocks over thousands of years and release nutrients into the soil.
We Need Oxygen To SurviveLast are the organisms of the world. They use oxygen in many forms. Their role in the cycle begins with carbon dioxide in the atmosphere. Plants take in that carbon dioxide and combine it with water to create sugars and oxygen molecules. Animals breathe that oxygen and both plants and animals use the sugars for energy. Through the process of metabolism, the sugars are broken down into water and carbon dioxide. Then the cycle begins again.
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Science Nation: Dead Zones in the Ocean (US-NSF Video)
Useful Reference MaterialsEncyclopedia.com (Nutrient Cycles):