Symbiosis (from ancient Greek σύν sýn , German 'together' and ancient Greek βίος bíos , German 'life' ) describes the socialization of individuals of two different species , which is advantageous for both partners.
Based on his work on lichens , Anton de Bary proposed in 1878 at the 51st meeting of German naturalists and doctors in Kassel that the term symbiosis should be introduced into biology for any coexistence of different organisms , including parasitism . In this broad sense, the term symbiosis ( English symbiosis ) is still used in US American literature for all forms of co-evolutionary coexistence, from mutualism to commensalism , neutralism and parasitism. In Europe, however, the term symbiosis is used in the narrower sense defined at the beginning.
Frequency of symbioses
Most of the biomass on earth consists of symbiotic systems as a large part of the trees and shrubs rely on pollination by other species. Then there are lichens , a symbiotic community between a fungus and green algae or cyanobacteria . Many sessile marine invertebrates living in shallow water, such as fire corals , most flowering animals and giant clams, live together with photosynthetic zooxanthellae . Another example are the stomach and intestinal bacteria of the animals, which enable cellulose-rich plant food to be broken down , for example in ruminants .
Differentiation according to the degree of mutual dependence
A differentiation between different forms of symbiosis results from the degree of mutual dependence of the species involved:
- Protocooperation (alliance): loosest form of symbiosis: both types benefit from living together, but are nonetheless viable without each other.
- Mutualism : Regular, but not vital, relationship between symbionts.
- Eusymbiosis, also obligatory symbiosis ( ancient Greek εὖ eu "good, real"): In eusymbiosis, the partners are no longer viable alone. Leaf-cutter ants cultivate mushrooms in their burrows and feed on them; the mushrooms, in turn, cannot multiply without the ants.
Differentiation on the basis of spatial relationship
A distinction between different forms of symbiosis results from the spatial or physical relationship of the two species involved:
- Endosymbiosis : one of the partners (endosymbiont) is absorbed into the body of the other (host). Examples are certain enterobacteria in the intestines of humans and animals, nodule bacteria in the roots of legumes , zooxanthellae in the reef-building stony corals of the tropical belt .
- Endocytobiosis : An organism (such as a bacterium or virus) that lives in the cells of other organisms or multiplies there (symbiotic or parasitic). See also § Endosymbiotic Theory .
- Exosymbiosis: The partners are only in contact with one another through their surface. Examples of this is the lichen symbiosis and Epixenosomen (to the Verrucomicrobia bacteria belonging) of the eyelash animalcule Euplotidium . Compare also parabiosis and epibiont (with the special case epiphyte ).
- Ectosymbiosis : The partners in a symbiosis remain physically separate (e.g. flowers and their pollinators; clownfish and their sea anemones).
A slightly different subdivision can be found in Ebert and Rühle (2009–2013): The authors differentiate between extracellular exosymbiosis, extracellular endosymbiosis, intracellular symbiosis and intranuclear symbiosis.
Differentiation according to the type of benefit achieved
A distinction between forms of symbiosis arises on the basis of the type of benefit achieved for the two species involved.
- Reproductive symbiosis: An example of reproductive symbiosis is the symbiosis between bees and flowering plants. The bee takes in the nectar of the flowers as food, while the pollen from the flower stick to it, which the bee then carries and pollinates another flower so that it can multiply. This is called zoophilia and is the "normal" act of pollination of flowering plants ( angiosperms ) by insects or birds, whereby the insects or birds receive nectar , but also pollen as food.
- Symbiosis to protect against enemies: An example of this symbiosis is the relationship between ants and aphids. The ants give the aphids protection from enemies, in return they let the ants "milk" them, they secrete a sugar solution which the ants ingest.
- Transport of plant seeds by animals, whereby animals eat the fruit and excrete the seeds again in another place ( zoochory ) or the seeds temporarily adhere to animals ( called animal litter ).
- Lichen is made up of algae and fungi, whereby the algae produce carbohydrates through photosynthesis , which are ingested by the fungi, while the fungi provide the algae with water and nutrient salts.
- Some green sulfur bacteria form microbial consortia (cell aggregates) with flagellated heterotrophic bacteria .
- With some species of ants, such as the leaf cutter ants , real mushroom farms are set up within the ant burrows, in which certain fungi are fertilized with plant residues and the spores of harmful molds are cleaned. Parts of the mushrooms serve as food for the ants. This form of symbiosis is called myrmecophilia .
- Mycorrhizal fungi remove carbohydrates from trees or other photosynthetic plants and in return supply minerals and water from the soil. Mycorrhiza is mandatory for all orchids , but also for many other plant species.
- The skin parasites of large mammals (e.g. hippopotamus and elephant) are eaten by cleaning birds, the same phenomenon occurs with cleaner fish , which attach themselves to large fish (e.g. sharks) and eat parasites from their skin (→ cleaning symbiosis ).
- In Yellowstone National Park in North America, a symbiosis between three participants has been demonstrated, a bluegrass , a mold and a virus . There are many hot springs there , around which the ground is heated. The grass Dichanthelium lanuginosum still tolerates temperatures of almost 70 ° C in the root area due to a symbiosis with the fungus Curvularia protuberata . Both the mushroom and the grass alone can only survive around 38 ° C. The third party involved, the CthTV virus ( Curvularia thermal tolerance virus ), which attacks the mold, is absolutely necessary in this symbiosis . If this virus is removed, the mold loses its heat resistance, and with it the grass in the hot locations perishes.
- Kappa-organisms are endosymbiotic bacteria in certain lines of paramecium Paramecium .
- Sulphide- oxidizing chemoautotrophic bacteria live as endosymbionts within the cells of polychaetes or between the cells of oligochaetes and as ectosymbionts on the surface of single cell colonies such as Zoothamnium niveum . This way of life gives them optimal conditions within the sulphide-rich milieu at hydrothermal springs such as the black smokers in the deep sea or in the vicinity of decaying organic substances in shallow water and are partially digested by their hosts. This symbiosis is so close in the beard worms (e.g. Riftia ) that the animals have no mouth opening when fully grown and no longer take in external food.
- Hermit crabs occasionally live in symbiosis with a sea anemone that has settled on its shell: the sea anemone protects the hermit crab from predators with its nettle cells ; the hermit crab "transports" the sea anemone to new feeding grounds, and the sea anemone also gets some of the hermit crab's prey.
- Plants can ingest endophytic bacteria, e.g. B. the nasturtium .
The endosymbiotic theory states that the mitochondria and chloroplasts ( cell organelles in eukaryotes - plants, animals and fungi) originated from endosymbiotic prokaryotes (aerobic, chemotrophic alphaproteobacteria or photosynthetically active, autotrophic , cyanobacteria ) at an early stage of evolution . This is supported by the similarities in the structural structure and in the biochemical characteristics which differ from the host cells but which correspond to the prokaryotes. Examples are its own DNA and the structure of the ribosomes , if present - even if the DNA is completely lost, genes of alphaproteo or cyanobacterial origin can still be found in the cell nucleus. Furthermore, these cell organelles multiply by dividing, just like bacteria do.
The inclusion of endosymbionts is an example of how symbiotic communities can become so close in the course of evolution that it makes sense to view them as newly formed biological species. This creation of a new species through the fusion of symbionts is called symbiogenesis . The importance of symbiogenesis was strongly emphasized in the 1970s by the American evolutionary biologist Lynn Margulis . According to their theory (which is highly controversial in its details, but largely accepted), symbiogenesis is one of the most important species-forming mechanisms of all.
For the scientific description and modeling of symbiotic systems, systems of ordinary differential equations, but occasionally more complex mathematical structures, are used in biology.
For example, with the aid of some idealizing simplifications, symbioses of two species on the level of population dynamics are described by:
if there is an effect of the symbiosis in a change in the intrinsic growth rate of the populations involved
if the primary effect is to adjust capacities.
(Designations: X, Y abundances of the species; a, b intrinsic growth rates of the species, K1, K2 capacities, cd ecological interaction parameters)
Mixed forms of these two simplifying borderline cases are of course possible and can normally be assumed in nature.
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