Autotroph

Any organism that produces complex organic compounds from abiotic substances present in its surroundings

An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,[1] generally using energy from light (photosynthesis) or inorganic chemical reactions (chemosynthesis).[2] They convert an abiotic source of energy (e.g. light) into energy stored in organic compounds, which can be used by other organisms (e.g. heterotrophs). Autotrophs do not need a living source of carbon or energy and are the producers in a food chain, such as plants on land or algae in water (in contrast to heterotrophs as consumers of autotrophs or other heterotrophs). Autotrophs can reduce carbon dioxide to make organic compounds for biosynthesis and as stored chemical fuel. Most autotrophs use water as the reducing agent, but some can use other hydrogen compounds such as hydrogen sulfide.

The primary producers can convert the energy in the light (phototroph and photoautotroph) or the energy in inorganic chemical compounds (chemotrophs or chemolithotrophs) to build organic molecules, which is usually accumulated in the form of biomass and will be used as carbon and energy source by other organisms (e.g. heterotrophs and mixotrophs). The photoautotrophs are the main primary producers, converting the energy of the light into chemical energy through photosynthesis, ultimately building organic molecules from carbon dioxide, an inorganic carbon source.[3] Examples of chemolithotrophs are some archaea and bacteria (unicellular organisms) that produce biomass from the oxidation of inorganic chemical compounds, these organisms are called chemoautotrophs, and are frequently found in hydrothermal vents in the deep ocean. Primary producers are at the lowest trophic level, and are the reasons why Earth sustains life to this day.[4]

Most chemoautotrophs are lithotrophs, using inorganic electron donors such as hydrogen sulfide, hydrogen gas, elemental sulfur, ammonium and ferrous oxide as reducing agents and hydrogen sources for biosynthesis and chemical energy release. Autotrophs use a portion of the ATP produced during photosynthesis or the oxidation of chemical compounds to reduce NADP+ to NADPH to form organic compounds.[5]

The term autotroph was coined by the German botanist Albert Bernhard Frank in 1892.[6][non-primary source needed] It stems from the ancient Greek word τροφή (trophḗ), meaning "nourishment" or "food". The first autotrophic organism developed about 2 billion years ago.[7] Photoautotrophs evolved from heterotrophic bacteria by developing photosynthesis. The earliest photosynthetic bacteria used hydrogen sulphide. Due to the scarcity of hydrogen sulphide, some photosynthetic bacteria evolved to use water in photosynthesis, leading to cyanobacteria.[8]

Some organisms rely on organic compounds as a source of carbon, but are able to use light or inorganic compounds as a source of energy. Such organisms are mixotrophs. An organism that obtains carbon from organic compounds but obtains energy from light is called a photoheterotroph, while an organism that obtains carbon from organic compounds and energy from the oxidation of inorganic compounds is termed a chemolithoheterotroph.

Evidence suggests that some fungi may also obtain energy from ionizing radiation: Such radiotrophic fungi were found growing inside a reactor of the Chernobyl nuclear power plant.[9]

Flowchart to determine if a species is autotroph, heterotroph, or a subtype

There are many different types of primary producers out in the Earth's ecosystem at different states. Fungi and other organisms that gain their biomass from oxidizing organic materials are called decomposers and are not primary producers. However, lichens located in tundra climates are an exceptional example of a primary producer that, by mutualistic symbiosis, combine photosynthesis by algae (or additionally nitrogen fixation by cyanobacteria) with the protection of a decomposer fungus. Also, plant-like primary producers (trees, algae) use the sun as a form of energy and put it into the air for other organisms.[3] There are of course H2O primary producers, including a form of bacteria, and phytoplankton. As there are many examples of primary producers, two dominant types are coral and one of the many types of brown algae, kelp.[3]

Gross primary production occurs by photosynthesis. This is also a main way that primary producers take energy and produce/release it somewhere else. Plants, coral, bacteria, and algae do this. During photosynthesis, primary producers take energy from the sun and produce it into energy, sugar, and oxygen. Primary producers also need energy to convert this same energy elsewhere, so they get it from nutrients. One type of nutrient is nitrogen.[4][3]

Without primary producers, organisms that are capable of producing energy on their own, the Earth would be unable to sustain itself.[3] Plants, along with other primary producers, produce the energy that beings consume, and the oxygen that they breathe.[3] It is thought that the first organisms on Earth were primary producers located on the ocean floor.[3]

Autotrophs are fundamental to the food chains of all ecosystems in the world. They take energy from the environment in the form of sunlight or inorganic chemicals and use it to create fuel molecules such as carbohydrates. This mechanism is called primary production. Other organisms, called heterotrophs, take in autotrophs as food to carry out functions necessary for their life. Thus, heterotrophs – all animals, almost all fungi, as well as most bacteria and protozoa – depend on autotrophs, or primary producers, for the raw materials and fuel they need. Heterotrophs obtain energy by breaking down carbohydrates or oxidizing organic molecules (carbohydrates, fats, and proteins) obtained in food.[10] Carnivorous organisms rely on autotrophs indirectly, as the nutrients obtained from their heterotrophic prey come from autotrophs they have consumed.

Aquatic algae are a significant contributor to food webs in tropical rivers and streams. This is displayed by net primary production, a fundamental ecological process that reflects the amount of carbon that is synthesized within an ecosystem. This carbon ultimately becomes available to consumers. Net primary production displays that the rates of in-stream primary production in tropical regions are at least an order of magnitude greater than similar temperate systems.[15]