Important introduction
In common language, the term symbiont refers to a microorganism that shares its life with another, while both derive benefits and mutual benefits from this union; in reality, this definition refers to a very specific type of symbiosis, defined mutualistic. Metaphorically speaking, the popular expression "living in symbiosis", entered into the common language, expresses the strong bond that unites two people: clearly, in jargon, the "symbiosis" identifies a hyperbole, an exaggeration in the context of a description of reality through phrases that amplify the concept considerably.
In biological terms, the symbiotic organisms live (literally) together: the word symbiosis, in fact, derives from the Greek sýn-bíōsis, which means "life together / cohabitation". This relationship can be advantageous for only one part or both, causing harm to an organism or being 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 the obligatory and the optional relationships.
Optional symbionts are organisms that can - even if not necessarily - live together to benefit each other: in this second category, organisms can also lead an independent life.
Classification
Furthermore, symbiotic relationships can be classified into several sub-categories; let's see, now, the most important ones:
- Mutualistic symbiosis or mutualism : this is a close correlation between people, objects or different 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 ) perform their biological activity by fixing nitrogen at the level of the leguminous root system: however, these microorganisms are able to reproduce even without interaction with the above-mentioned plants. At first glance, the statement " man lives in mutual symbiosis with some bacteria " may be bizarre: however, this expression, carefully observed, is not so strange. Just think of the microorganisms of the intestinal bacterial flora which, living in the intestine of man, can survive by ensuring (as a thank you) the intestinal balance of the host. Among other outstanding examples of symbiotic relationship, we recall 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.
- Parasitism : parasitism is a form of symbiosis in which the protagonists of the relationship do not benefit from each other: in other words, an organism benefits from the other. The symbionts in question are defined, precisely, "parasite" and "host": the parasite, without independent life, is generally smaller than the host, has a much shorter life span and can live only if related to the other symbiont. To clarify the concept, we report some simple examples: the parasites par excellence are the bacteria, the viruses and the fungi, which infect the man (guest). However, among the "parasitic symbionts" we also mention some crustaceans, insects and flowering plants. Again, it is good to distinguish two categories of parasitic symbionts: the ectoparasites live on the host's surface, while the endoparasites are related to the other symbiont living inside it.
- Commensalism : commensalism is another form of symbiosis, in which one organism benefits from the relationship, while the other living being (also called symbiont) is neither damaged nor helped. In this symbiosis, the components are optional organisms, in which the strongest exploits the other without the latter being able to benefit from the relationship.
- Tenantism : it is a form of commensal symbiotic relationship, in which the two protagonists of the relationship do not necessarily depend on each other, but one takes advantage of the other, without creating damage or benefit. It is the case of plants like the orchids that live on trees, as well as some animals that lodge in the hollows of trees.
- Amensalism : omnipresent in the natural world, amensalism is a form of symbiosis in which an organism of the relationship is completely annulled, while the other remains unaltered, without benefiting or disadvantaging. 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; 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 speak 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.
Conclusions
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 among the symbiotic partners could have escaped the other.
