The Importance of Oxygen

After hydrogen and helium, oxygen is the third most abundant element in the universe. In stars such as our sun, its formation is part of the fuel cycle that provides the star’s energy. On planet Earth, oxygen is the most abundant element:

• 85.7% by weight in seawater (88.8 % of pure water is oxygen)
• 46.6% by weight in the Earth’s crust
• 20.95% by volume in the lower atmosphere

Compared with the other planets in our solar system, Earth has a remarkable level of oxygen gas in the lower atmosphere. The abundance of oxygen is one of the main reasons our planet supports life as we know it. Oxygen is essential to numerous biological systems and processes; among other things, oxygen is:

• A key element in the proteins, sugars, nutrients, and other chemicals that sustain life
• An essential element in deoxyribonucleic acid (DNA)
• Crucial for biochemical processes such as photosynthesis

Oxygen circulates through the ecosystem and the biosphere in various forms. It is free in the air and dissolved in water. Certain lower-order plants and all animals use oxygen to respire and return it to air and water as carbon dioxide (CO₂). CO₂ is then taken up by algae and terrestrial green plants and converted into carbohydrates during the process of photosynthesis, combining it with water to create sugars and oxygen molecules. Free oxygen is returned to the air, and the cycle starts again. Almost all the free oxygen present in the atmosphere is the result of photosynthesis. The waters of the world are the main sources of atmospheric oxygen; their algae are estimated to replace about 90% of all oxygen used.