Diatomic oxygen (O2) is a highly reactive, paramagnetic molecule (containing two unpaired electrons) that occurs as a colorless gas at ambient temperatures but is a sapphire-blue liquid at very low temperatures (below –183 ºC). It is the second most abundant gas present in Earth's atmosphere. But it wasn't that way 4.5 billion years ago when the Earth's atmosphere was forming and N2 and CO2 were the dominant atmospheric gases. During that time, our planet was essentially devoid of oxygen, except for a small amount formed from the photodissociation of water and carbon dioxide by ultraviolet light from the sun. The oxygen that we breath today started to enter the atmosphere as a byproduct of photosynthesis carried out by early cyanobacteria 2.5 billion years ago. These bacteria thrived in the primordial oceans and were able to make organic nutrients using the light of the sun and the CO2 and water around them. During those early days, all the oxygen produced by bacteria was used up to oxidize iron that was dissolved in the oceans. It took about a hundred million years of oxygen production before all the iron precipitated, whereupon the oceans became saturated with oxygen and outgassed oxygen into the atmosphere. Today, oxygen continues to be produced photosynthetically by phytoplankton and green plants that have since evolved on Earth. Marine and terrestrial animals alike use the oxidizing power of dioxygen to pull electrons from organic molecules in electron transport systems that make up their metabolisms (aerobic respiration). Related to this is the best known reaction of diatomic oxygen: the reaction of O2 with the protein hemoglobin that that is responsible for oxygen transport in our blood.
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