Symbiosis and Symbiosis

Important premise

In common parlance, the term symbiont it refers to a microorganism that shares its life with another, while both derive mutual benefits and advantages from this union; in reality, this definition refers to a very specific type of symbiosis, defined as mutualistic. Metaphorically speaking, the popular expression "living in symbiosis", entered into common language, expresses the strong bond that unites two people: clearly, in jargon, "symbiosis" identifies a "hyperbole, an" exaggeration in the context of a description of reality through phrases that amplify the concept considerably.
In biological terms, symbiotic organisms live (literally) together: the word symbiosis, in fact, derives from the Greek sýn-bíōsis, which means "life together / coexistence". Such a relationship can be beneficial to one party or both, harm one organism or be harmless to both.

Symbiosis and symbiosis

The symbiotic relationships between the various living organisms are not all the same: first of all, a clear distinction must be drawn between obligatory and optional relationships.

In obligate symbiosis, symbiotic organisms depend on each other, and their survival is strongly conditioned by their union: in other words, the cessation of symbiotic life for these microorganisms would lead to the death of both. Just think, for example, of the need for symbiotic life between photosynthetic microorganisms (eg cyanobacteria or algae) and fungi: lichens are in fact defined as symbiotic microorganisms formed by these two components, and the absence of one implies the death of the other. .

Optional symbionts are organisms that can - although not necessarily must - live together for mutual benefit: in this second category, organisms can also lead an independent life.


Furthermore, symbiotic relationships can be classified into several sub-categories; now let's see the most important:

  1. Mutualistic symbiosis or mutualism: this is a close correlation between different people, objects or actions, in order to gain mutual benefit. Probably, the mutualistic variant is the most widespread symbiosis of all, and involves the components of the whole living kingdom (including man): more precisely, it is the physical and biochemical relationships that lay the foundations for defining a symbiotic relationship or not. For example, some nitrogen-fixing bacteria (eg Gen. Rhizobium) carry out their biological activity by fixing nitrogen at the level of the root system of legumes: however, these microorganisms are able to reproduce even without the "interaction with the aforementioned plants. At first glance, the" affirmation "man lives in mutualistic symbiosis with some bacteria"may be bizarre: however, this expression, observed carefully, is not so strange. Just think of the microorganisms of the intestinal bacterial flora which, living in the" intestine of "man, can survive ensuring (as a thank you) the" intestinal balance of " host. Among other exceptional examples of symbiotic relationship, we remember the link between plants and fungi, as well as the union between bacteria and plants, between animals of different species (eg shark and pilot fish), between animals and fungi (eg. ants and mushrooms), etc ..
  2. Parasitism: parasitism is a form of symbiosis in which the protagonists of the relationship do not benefit from each other: or rather, an organism benefits at the expense of the other. The symbionts in question are precisely defined as "parasite" and "host" : the parasite, devoid of independent life, is generally smaller than the host, has a much shorter lifespan and can only live if related to the other symbiont. To clarify the concept, we report some simple examples: parasites for antonomasia are bacteria, viruses and fungi, which infect man (host). However, among the "parasitic symbionts" we also mention some crustaceans, insects and angiosperms. Again, it is good to distinguish two categories of parasitic symbionts: the ectoparasites live on the surface of the host, while the endoparasites they are related to the other symbiont living inside it.
  3. Commensalism: commensalism is another form of symbiosis, in which an organism benefits from the relationship, while the other living being (however called a symbiont) is neither damaged nor helped. In this symbiosis, the components are optional organisms, in which the stronger exploits the other without the latter being able to benefit from the relationship.
  4. Tenancy: it is a form of symbiotic commensal relationship, in which the two protagonists of the relationship do not necessarily depend on each other, but one benefits from the other, without creating harm or benefit. This is the case of plants such as orchids that they live in trees, as well as some animals that lodge in tree holes.
  5. Amensalism: omnipresent in the natural world, amensalism is a form of symbiosis in which one organism of the relationship is completely canceled, while the other remains unaffected, without benefiting or disadvantage. To give a practical example, just think of a mighty tree whose shadow covers and damages a tree or a smaller plant that grows near it: the mighty tree, with its shadow, prevents the seedling from absorbing sunlight; in the at the same time, the tree steals nutrients and rainwater from the other symbiont. If the plant dies, the largest tree can feed on the remains of its decomposition: in this case, we are speaking precisely of another type of symbiosis, parasitism. Here is another example: the Pennicillium, secreting penicillin (a bactericidal compound that is part of its natural metabolism) exerts a negative (toxic) effect on a second symbiont.


In the wonderful world of the living, symbiosis plays a role of absolute prestige, since all eukaryotic organisms - such as plants, animals, protists and fungi - seem to derive precisely from the symbiosis between various types of prokaryotes (bacteria). We are talking about the endosymbiotic theory, in which the close relationship, as well as union, between two and more prokaryotic organisms has inexorably led to the creation of increasingly complex life forms, up to the achievement of a permanent symbiosis in all respects, in which no one between the symbiotic partners could have withdrawn from the other.

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