Overview of cycle between autotrophs andheterotrophs.Photosynthesis is the main means by which plants, algae and many bacteria produce organic compounds and oxygen from carbon dioxide and water (green arrow).
Mostchemoautotrophs arelithotrophs, using inorganic electron donors such as hydrogen sulfide,hydrogen gas, elementalsulfur,ammonium andferrous oxide as reducing agents and hydrogen sources forbiosynthesis and chemical energy release. Autotrophs use a portion of theATP produced during photosynthesis or the oxidation of chemical compounds to reduceNADP+ to NADPH to form organic compounds.[5]
The termautotroph was coined by the German botanistAlbert Bernhard Frank in 1892.[6][7] It stems from the ancient Greek wordτροφή (trophḗ), meaning "nourishment" or "food". The first autotrophic organisms likely evolved early in the Archean but proliferated across Earth'sGreat Oxidation Event with an increase to the rate of oxygenicphotosynthesis bycyanobacteria.[8] Photoautotrophs evolved fromheterotrophic bacteria by developingphotosynthesis. The earliest photosynthetic bacteria usedhydrogen sulphide. Due to the scarcity of hydrogen sulphide, some photosynthetic bacteria evolved to use water in photosynthesis, leading tocyanobacteria.[9]
Some organisms rely onorganic compounds as a source ofcarbon, but are able to uselight orinorganic compounds as a source of energy. Such organisms aremixotrophs. An organism that obtains carbon from organic compounds but obtains energy from light is called aphotoheterotroph, while an organism that obtains carbon from organic compounds and energy from the oxidation of inorganic compounds is termed achemolithoheterotroph.
There are many different types of autotrophs in Earth's ecosystems.Lichens located in tundra climates are an exceptional example of a primary producer that, by mutualistic symbiosis, combines photosynthesis byalgae (or additionally nitrogen fixation by cyanobacteria) with the protection of a decomposerfungus. 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 the main way that primary producers get energy and make it available to other forms of life. Plants, many corals (by means of intracellular algae), some bacteria (cyanobacteria), and algae do this. During photosynthesis, primary producers receive energy from the sun and use it to produce sugar and oxygen.
Without primary producers, organisms that are capable of producing energy on their own, the biological systems of Earth would be unable to sustain themselves.[3] Plants, along with other primary producers, produce the energy that other living 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 allecosystems 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 calledprimary production. Other organisms, calledheterotrophs, take in autotrophs asfood to carry out functions necessary for their life. Thus, heterotrophs – allanimals, almost allfungi, as well as mostbacteria andprotozoa – depend on autotrophs, orprimary 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.Carnivorous organisms rely on autotrophs indirectly, as thenutrients 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 in similar temperate systems.[15]
Researchers believe that the first cellular lifeforms were not heterotrophs as they would rely upon autotrophs since organic substrates delivered from space were either too heterogeneous to support microbial growth or too reduced to be fermented. Instead, they consider that the first cells were autotrophs.[16] These autotrophs might have beenthermophilic andanaerobicchemolithoautotrophs that lived at deep sea alkaline hydrothermal vents. This view is supported by phylogenetic evidence – the physiology and habitat of thelast universal common ancestor (LUCA) is inferred to have also been a thermophilic anaerobe with a Wood-Ljungdahl pathway, its biochemistry was replete with FeS clusters and radical reaction mechanisms. It was dependent upon Fe, H2, and CO2.[16][17] The high concentration of K+ present within the cytosol of most life forms suggests that early cellular life hadNa+/H+ antiporters or possibly symporters.[18] Autotrophs possibly evolved into heterotrophs when they were at low H2 partial pressures where the first form of heterotrophy were likely amino acid and clostridial type purine fermentations.[19] It has been suggested that photosynthesis emerged in the presence of faintnear infrared light emitted by hydrothermal vents. The first photochemically active pigments are then thought to be Zn-tetrapyrroles.[20]
^Frank, Albert Bernard (1892–93).Lehrbuch der Botanik (in German). Leipzig: W. Engelmann.Archived from the original on 7 March 2023. Retrieved14 January 2018.
^Townsend, Rich (13 October 2019)."The Evolution of Autotrophs".University of Wisconsin-Madison Department of Astronomy.Archived from the original on 8 July 2022. Retrieved3 May 2019.
^Schurr, Sam H. (19 January 2011).Energy, Economic Growth, and the Environment. New York.ISBN9781617260209.OCLC868970980.{{cite book}}: CS1 maint: location missing publisher (link)
^Odum, Eugene P. (Eugene Pleasants), 1913-2002. (2005).Fundamentals of ecology. Barrett, Gary W. (5th ed.). Belmont, CA: Thomson Brooks/Cole. p. 598.ISBN0-534-42066-4.OCLC56476957.{{cite book}}: CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)
