Chemosynthesis is a process used by some autotrophs to produce food without sunlight, using the oxidation of inorganic chemicals. These organisms, including extremophiles, are found in a variety of environments, including hydrothermal vents, soil, and the gut of mammals. Chemosynthesis may have played a role in the creation of early life on Earth and could be used for fuel generation and waste detoxification. The ability of chemosynthetic organisms to thrive in extreme conditions has led to speculation about their existence on other planets.
Chemosynthesis is a process that some organisms use to obtain energy for food production, similar to photosynthesis, but without the use of sunlight. Energy comes from the oxidation of inorganic chemicals that organisms find in their environment. The process occurs in many bacteria and another group of organisms known as archaea. Life forms that use this method of obtaining energy are found in a variety of environments, including soil, the gut of mammals, oil deposits, and in extreme conditions, such as around hydrothermal vents on the ocean floor. They’re adapted to circumstances that may have been common billions of years ago, leading some scientists to theorize that they may be direct descendants of early life on Earth.
Methods
Organisms that make their own food with inorganic chemicals, instead of using already existing organic materials, are known as autotrophs. Foods are made up of carbohydrates, such as glucose, but these require energy to produce. Where sunlight is available, autotrophs will generally use it to perform photosynthesis, but in places where light doesn’t reach, different types have evolved that use chemical energy instead. The life forms that do this are known as chemautotrophs. Different methods have emerged, determined by the conditions and chemicals available.
Chemosynthesis uses redox reactions, also known as redox reactions, to provide the energy needed to produce carbohydrates from carbon dioxide and water. This type of reaction involves losing electrons from one substance and adding electrons to another. The substance that receives the electrons, usually oxygen, is said to have been reduced, while the substance that supplies them has been oxidized. Reduction requires energy, but oxidation releases it. The two reactions always occur together, but those used in chemosynthesis result in an overall release of energy.
As with photosynthesis, the actual reactions are very complex and involve a number of steps, but can be summarized in terms of raw materials and end products, one of which will be food in the form of some type of carbohydrate. Where sulfides are available, they can be oxidized, producing sulfur or sulfates. Iron can also be oxidized, from a form known as iron II to iron III, which has one electron missing. Methane, which occurs in some places as natural gas, can be a source of both energy and carbon for some microorganisms and is also a byproduct of chemosynthesis in other organisms. The oxidation of ammonia into nitrites and nitrates is another method that provides energy for some life forms.
Many of the organisms that use chemosynthesis to make food live in environments with extreme temperatures, pressures, salinity, or other conditions that are hostile to most life. These are known as extremophiles. They have various adaptations that allow them to survive, such as unusual enzymes that are not deactivated by high temperatures.
Environments
Hydrothermal vents are among the most extraordinary environments on the planet. They consist of streams of warm, chemical-rich water that spew from the ocean floor into geologically active areas, such as mid-ocean ridges. While seemingly hostile to life, with no light, temperatures approaching 212°F (100°C), and filled with chemicals toxic to most life forms, they have thriving and diverse ecosystems supported by chemosynthetic microorganisms. These microbes consist of bacteria, and also of archaea, a very ancient group of organisms that are superficially similar, but chemically and genetically very different.
The hot water produced by the hydrothermal vents is very rich in sulphides, which the microbes use for chemosynthesis, sometimes releasing methane as a by-product. The microorganisms that produce this gas are known as methanogens. Other chemosynthetic microbes in this environment obtain energy from the oxidation of methane, converting the sulfate to sulfide in the process. Methane oxidation also occurs in areas where oil, a mixture of hydrocarbons including methane, seeps upward to the seabed.
The ecologies surrounding deep-sea vents are much richer than those farthest from such chemical sources, which must subsist solely on dead organic matter that slowly descends from the waters above. Chemosynthetic life forms not only provide the foundation for larger communities of organisms that consume microbes for survival, but also form important symbiotic relationships with other organisms. An interesting example is the tube worm, which starts life with a mouth and gut, which it uses to absorb huge numbers of chemosynthetic bacteria. In a later stage it loses its mouth and continues to survive by consuming the food produced by its internal bacteria.
Extremophile chemosynthetic microorganisms have been found in hot springs, where they survive by oxidation of sulfur or ammonia, and in rocks deep below the surface, where they obtain energy by oxidizing iron. Chemosynthesis also occurs in more familiar places. For example, in soil, nitrifying bacteria convert ammonia into nitrites and nitrates, while methane-generating archaea are found in marshes and marshes, wastewater, and the intestines of mammals.
Importance and possible uses
Nitrifying bacteria in soil provide usable nitrogen for plants and are a crucial part of the nitrogen cycle – without them, plants and animals could not exist. It is very likely that early life forms used chemosynthesis to create organic compounds from inorganic ones, and thus these processes may be responsible for creating life on Earth. Scientists have suggested a number of ways in which chemautotrophs could be put to good use. For example, they could be used to generate methane for fuel. Because many of these organisms live off chemicals that are toxic to humans and release harmless byproducts, they could also be used to detoxify some types of poisonous waste.
Chemosynthesis and other planets
The ability of some chemosynthetic organisms to thrive in extreme conditions has led some scientists to suggest that such life forms could exist on other planets, in environments that would not be suitable for more familiar types of life. Experiments suggest that some chemosynthetic organisms may be able to survive and grow beneath the surface of Mars, and it has been speculated that trace amounts of methane found in the Martian atmosphere could be the result of the activity of methanogenic microorganisms. Another possible location for extraterrestrial life is Jupiter’s ice-covered moon Europa, where liquid water is thought to exist beneath the surface.
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