Oxygen is one of the most plentiful gasses in our atmosphere. While it is used by all organisms on earth, it is also replenished by the plant life on earth through photosysnthesis, so it is a renewable resource. There is disagreement over which photosynthetic organisms, land plants or phytoplankton and algae in the oceans, provide the majority of the earth's photosynthesis.
Oxygen makes up about 21% or 210,000 ppm (particles per million) of the atmosphere. Atmospheric carbon dioxide is now at about 365 ppm, and Salisbury and Ross (1985) state that only about 10% of the carbon dioxide in the atmosphere is used each year in photosynthesis. If that figure is a good estimate then you can see that a very tiny percentage of the earth's total atmospheric oxygen is produced each year by photosynthesis, i.e. 36.5 ppm is only 0.017% of total atmospheric oxygen.
Photosynthesis Photosynthesis equation
But, how was the Oxygen in the atmosphere formed initially?
Gaseous oxygen was created on the Earth about 3.5 billion years ago. At that time, the UV light of the sun decomposed the water vapour molecules present in the atmosphere, and released oxygen and hydrogen. But a major part of the oxygen immediately reacted with other substances, so that it was no longer available in the atmosphere as gas. Oxygen was also produced by the seas, where the ‘blue algae’ carried out photosynthesis. These bacteria converted sun light and carbon dioxide into energy. In the process, gaseous oxygen was released as a ‘waste product, which accumulated in the atmosphere. About one billion years ago, about one-twentieth (4%) of the atmosphere consisted of oxygen.