Where does the world oxygen come from?« Back to Questions List

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 Atom


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.

oxygen2                                 oxygen3

Photosynthesis                                                                                Photosynthesis  equation

Right now, of course, the earth's oxygen level is falling every year so slightly as the carbon dioxide concentration increases about 1.5 parts per million (ppm)each year. The carbon dioxide increase/oxygen decrease is due to deforestation and burning of fossil fuels. Thus, we are using some of the atmospheric oxygen added by ancient photosynthesis. Fossil fuels, such as coal, oil and natural gas represent  carbon fixed by ancient photosynthesis. Fossil fuels can also be considered to represent photosynthetic oxygen placed in the atmosphere, tens-of-millions to hundreds-of-millions of years ago. Therefore, you can consider that you are breathing some oxygen originally added to the atmosphere by giant lycopods, tree ferns and giant horsetails from the Carboniferous era 360 to 286 million years ago.  


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.