Dinopedia
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Oxygen is a chemical element with symbol O and chemical number 8.  Many organic molecules such as proteins, carbohydrates, lipids, and nucleic acis contain oxygen as well as inorganic molecules such as animal shells, teeth, and bone.  Oxygen is the third most abundant element in the universe after hydrogen and helium.  It makes up 21% of Earth's atmosphere as dioxygen.  Oxygen is produced by cyanobacteria, algae, and plants, and cyanobacteria was responsible for the Oxygen Catastrophe that wiped out many obligate anaerobic bacterias in the Precambrian.

Oxygen
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