Oxygen is an important constituent of air around us. Approximately 21% of the atmosphere is comprised of oxygen. Most of the oxygen found in our atmosphere is released by plants, as a bi-product of photosynthesis. Oxygen is highly reactive. As the oxygen in our atmosphere interacts with other substances, it often bonds to them, becoming trapped. Many life forms also remove oxygen from the atmosphere, as they breathe. This oxygen is used by these life forms to carry out the functions of life.
The Oxygen cycle is the biogeochemical cycle that describes the movement of oxygen within its three main reservoirs: the atmosphere, biosphere, and lithosphere.
Silicate and oxide minerals of the Earth’s crust and mantle are the largest reservoir of oxygen (99.5%). Only a small portion has been released as free oxygen to the biosphere (0.01%) and atmosphere (0.36%). Photosynthesis is the main source of atmospheric oxygen, which produces sugars and oxygen from carbon dioxide and water:
An additional source of atmospheric oxygen comes from photolysis whereby high energy ultraviolet radiation breaks down atmospheric water and nitrous oxide into component atoms. The free H and N atoms escape into space leaving O2 in the atmosphere:
Process of respiration and decay remove oxygen from atmosphere. During these mechanisms animals and bacteria consume oxygen and release carbon dioxide.
Marine organisms create calcium carbonate shell material (CaCO3) that is rich in oxygen. When the organism dies its shell is buried over time to create the limestone rock of the lithosphere. Weathering processes initiated by organisms can also free oxygen from the lithosphere. Plants and animals extract nutrient minerals from rocks and release oxygen in the process.
Image source: https://en.wikipedia.org/wiki/File:Oxygen_Cycle.jpg
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