The origin and geological history of oxygen
Oxygen is the third most profusely found element in the universe. It is naturally found in the sun and plays a significant role in the carbon cycle. Being the primary member of Group 16 of the periodic table, oxygen is a chemically active element, forming compounds with nearly all the elements except the inert gases. It is paramagnetic in all its three forms; i.e., solid, liquid, and gaseous state.
Oxygen is denser than air and can be dissolved in water up to an explicit extent. Commercially, oxygen can be prepared by the process of liquefaction and fractional distillation of air and through electrolysis of water.
Oxygen combines directly with various elements to form oxides. Oxygen is present as important constituent of many acids, hydroxides, and various other compounds. A vital feature of oxygen includes when cooled below its boiling point, oxygen turns into a pale blue liquid and when cooled even more, the liquid solidifies while retaining its color.
Oxygen is a poor conductor of heat and electricity. The natural oxygen in the atmosphere called diatomic oxygen gas has a molecular mass of 32 while ozone (O3), which is more reactive than natural oxygen, is another allotrope of oxygen formed due to electrical discharges or ultraviolet light reacting with the atmospheric oxygen. In its molecular form, oxygen is found almost anywhere in the atmosphere.
Oxygen is regarded as highly reducing gas. It tends to combine with other molecules like atmospheric gases or surface rocks. It is the second-most electronegative atom in the periodic table after fluorine. Oxygen has a strong tendency to rip electrons from other atoms. As a consequence, any given oxygen molecule has a relatively short lifetime in the atmosphere.
History of oxygen evolution
Prior to 3.45 billion years ago, Earth’s atmosphere and oceans were anoxic (i.e. without oxygen). This is supported by the existence of mass-independent fractionalization (MIF) of sulfur isotopes in sediments from this time period, for these can only form in the absence of oxygen.
Then, during the periods between 2.45 and 1.85 billion years ago, molecular oxygen appeared. Photosynthesis is the main process for plant through which some amounts of oxygen get released into the environment. Other than plants few kinds of bacteria called cyanobacteria also able release oxygen.
Oxygen levels have since continued to more-or-less rise, peaking at 30 per cent of the total atmospheric content during the Carboniferous era some 350 million years ago. During this time, the burial rate of organic matter was rapid, preventing oxygen from combining with carbon in dead organisms and keeping it in the atmosphere. The high availability of oxygen during this period may explain the enormous insects of the Carboniferous era. Oxygen also plays vital role in animals. If more oxygen is available to the absorbed into the animal blood, then blood may deliver the oxygen further into different parts of body, thereby oxygen supporting larger body structures in animals.
Several models assume that during the Proterozoic Eon (Proterozoic Eon era ranges from 2.5 billion to 542 million years ago), the concentration of oxygen in the ocean was substantially lower than atmospheric oxygen levels.
Oxygen evolution is the process of generating molecular oxygen through chemical reaction. Mechanisms of oxygen evolution include the oxidation of water during oxygenic photosynthesis, electrolysis of water into oxygen and hydrogen, and electrocatalytic oxygen evolution from oxides and oxoacids.
Photosynthetic oxygen evolution occurs via the light-dependent oxidation of water to molecular oxygen and can be written as the following simplified chemical reaction
2H2O → 4e-+ 4H++ O2
The reaction requires the energy of four photons. The electrons from the oxidized water molecules replace electrons in the P680 component of photosystem II that have been removed into an electron transport chain via light-dependent excitation and resonance energy transfer onto plastoquinone. Photosytem II, therefore, has also been referred to as water-plastoquinone oxido-reductase.
The protons are released into the thylakoid lumen, thus contributing to the generation of a proton gradient across the thylakoid membrane. This proton gradient is the driving force for ATP synthesis via photophosphorylation and coupling the absorption of light energy and oxidation of water to the creation of chemical energy during photosynthesis.
Oxygen evolution occurs as a byproduct of hydrogen production via electrolysis of water. While oxygen production is not the main focus of industrial applications of water electrolysis, it becomes essential for life support systems in situations that require the generation of oxygen for air revitalization.
Human exploration of regions that lack breathable oxygen, such as the deep sea or outer space, requires means of reliably generating oxygen apart from earth’s atmosphere. Submarines and spacecraft utilize either an electrolytic mechanism (water or solid oxide electrolysis) or chemical oxygen generators as part of their life support equipment.
Rising oxygen concentrations have been cited as a driver for evolutionary diversification, although the physiological arguments behind such arguments are questionable, and a consistent pattern between oxygen levels and the rate of evolution is not clearly evident.
There are three hypothetical theories for oxygen evolution
Oxygen reacted with iron in seawater, and the resulting iron oxide precipitated onto the seafloor, and then was buried deep within the Earth.
Oxygen-rich water in seafloor sediments was buried within the Earth, leaving oxygen in the mantle when the water’s hydrogen was belched out by volcanoes.
Oxygen-rich sulfates in undersea hot springs reacted with iron in seafloor sediments, which were buried to put oxygen into the mantle.
According to carbon-burial theory when organic material is buried, oxygen becomes available to build up in the atmosphere. So there was a sudden increase 2.3 billion years ago in the amount of organic carbon that was buried, leaving more free oxygen.
Oxygen prevents growth of the most primitive living bacteria such as photosynthetic bacteria, methane-producing bacteria and bacteria that derive energy from fermentation.
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