The cycle of oxygen in nature

It is officially recognized that in 1774 the English chemist Joseph Priestley discovered oxygen (O2). As a result of the experiment carried out in a closed vessel with mercury oxide, under the influence of the sun rays directed by the lens, its decomposition took place: 2HgO → O2 ↑ + 2Hg. This gaseous substance is characterized by a density under normal conditions of 0.00142897 g / cm³, a molar volume of 14.0 cm³ / mole, a melting point of minus 218.2 ° C and a boiling point minus 182.81 ° C. The molar mass is 15.9994 g / mol. The main characteristic of oxygen is its ability to oxidize various substances. Being an active non-metal, O2 interacts with all metals with the formation of basic and amphoteric oxides, as well as with all non-metals (except halogens), resulting in acidic or non-salt-forming oxides.

Oxygen is part of more than one and a half thousand substances, since it is the most common chemical element on Earth. It is a part of various chemical compounds (there are more than one and a half thousand). In the solid crust, the O2 content is 47.4%. In marine and fresh waters , its share in the bound state accounts for 88.8% of the mass. In the atmosphere, oxygen is in a free state, its volume fraction is approximately 21%, and its mass fraction is 23.1%. It is the most important constituent of organic substances that are present in every living cell. By volume in them it occupies 25%, and by weight 65%. The cycle of oxygen in nature is due to its chemical activity.

A cycle is a series of changes in a substance, as a result of which it returns to the starting point, and the whole path is repeated. The oxygen cycle is a biogeochemical movement. Through it, O2 passes through the biotic sum of all ecosystems (biosphere or life zone on Earth) and abiotic (lithosphere, atmosphere and hydrosphere) environment. The cycle of oxygen describes its movement in the hydrosphere (the mass of water that is underground and above its surface), the atmosphere (air), the biosphere (the global sum of all ecosystems) and the lithosphere (the earth's crust). Violations of this cycle in the hydrosphere can lead to the development of hypoxic (low O2) zones in large lakes and the ocean. The main driving factor is photosynthesis.

Ecological systems (ecosystems) have many biogeochemical cycles operating in their composition. For example, the water cycle, the circulation of oxygen, the cycle of nitrogen, carbon, etc. All chemical elements pass the path, which is part of biogeochemical cycles. They are an integral part of living organisms, but also move through the abiotic environments of ecosystems. This water (hydrosphere), the earth's crust (lithosphere) and air (atmosphere). Living organisms fill the Earth's shell, called the biosphere. All nutrients, such as carbon, nitrogen, oxygen, phosphorus and sulfur, are used by them and are part of a closed system, so they are recycled, not lost and not replenished constantly, as in an open system.

The largest reservoir of O2 (99.5%) is the crust and mantle of the Earth, where it is contained in silicate and oxide minerals. The oxygen cycle provided only a small fraction of free O2 into the biosphere (0.01%) and into the atmosphere (0.36%). The main source of atmospheric free O2 is photosynthesis. Its products are organic substances and free oxygen, formed from carbon dioxide and water: 6CO2 + 6H2O + energy → C6H12O6 + 6O2.

The terrestrial plants, as well as the phytoplankton of the oceans, respond to the cycle of oxygen in the biosphere. Tiny sea cyanobacteria (blue-green algae) Prochlorococcus, 0.6 microns in size, were discovered in 1986. They account for more than half of the products of photosynthesis in the open ocean. An additional source of free atmospheric oxygen is the phenomenon of photolysis (a chemical reaction proceeding under the action of photons). As a result, atmospheric water and nitrous oxide dissociate into constituent atoms, hydrogen (H) and nitrogen (N) remove space, and O2 remains in the atmosphere: 2H2O + energy → 4H + O2 and 2N2O + energy → 4N + O2. The free oxygen of the atmosphere is consumed by living organisms in the processes of respiration and decay. The lithosphere uses free O2 as a result of chemical weathering and surface reactions. For example, it is expended on the formation of iron oxides (rust): 4FeO + O2 → 2Fe2O3 or oxides of other metals and nonmetals.

The cycle of oxygen also includes a cycle between the biosphere and the lithosphere. Marine organisms in the biosphere serve as sources of calcium carbonate (CaCO3), which is rich in O2. When the body dies, its shell is carried out in the shallow waters of the seabed, where it is located for a long time and forms limestone (a sedimentary rock of the earth's crust). The weathering processes initiated by the biosphere can also extract free oxygen from the lithosphere. Plants and animals extract nutrients from sedimentary rocks and release oxygen.