Theory:

Atmospheric air contains \(21\%\) of oxygen (O2). The biogeochemical process, which explains the oxygen circulation, is known as the oxygen cycle. Oxygen (O2) is the second most abundant gas in the atmosphere. Aquatic organisms breathe oxygen dissolved in water.
  
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We know that oxygen and carbon cycles are connected, for example, photosynthesis. The following is the process involved in the oxygen cycle
 
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  • In the respiration process, atmospheric oxygen is used.
  • During photosynthesis, O2 is released as a byproduct back to the atmosphere.
This process again repeats. Oxygen is the essential component of carbohydrates, proteins, facts etc., present in the human body.
Fact: \(65%\) of oxygen is present in the human body.
The main hoard of oxygen is present in the atmosphere, biosphere, lithosphere, and hydrosphere. The lithosphere is the largest oxygen reservoir on earth; for example, silica (SiO2) contains a large amount of oxygen and other minerals. The oxides of metals found on the earth also contain oxygen.
 
The natural process that produces oxygen is by photosynthesis and the reaction of sunlight with water vapour present in the atmosphere (photolysis). There is also some chemical weathering process that releases oxygen, for example, rusting.
 
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We also know that shells of marine organisms are made of calcium carbonate (CaCO3), so when they die, they get settled at the seafloor and form limestone sedimentary rock. Thus, the weathering of these rocks and the decomposition process of marine organisms release oxygen back to the lithosphere.
 
The atmospheric oxygen is also essential for combustion, respiration, decomposition and rusting.