SYMBIOSIS. - From the Greek σύν (= together) and βίος (= life). This term, introduced by A. de Bary in 1879, denotes the intimate and constant association of two animal or plant organisms of different species, such as to create between them reciprocal relationships that enable them to attain more favorable conditions of life.
When both components of a symbiosis (symbionts) derive equal benefit from their association, one speaks more precisely of mutualism or mutualistic symbiosis. From this condition one passes gradually to others, in which the equilibrium of the symbiosis is less perfect, so that one of the two symbionts derives a more notable advantage from the association than the other. Thus one arrives at cases of simple inquilinism, in which one species lives as a lodger on another, occupying a part of its body without causing it either harm or advantage; or at those of commensalism, in which one species lives together with another for the sole purpose of feeding on the leftovers of its meal. Quite different are the conditions of parasitism (v.), in which a parasitic species harms or slowly kills the involuntary host that shelters it, deriving from it nourishment and all the other conditions of existence. Some hold that certain forms of symbiosis or inquilinism represent a preparatory stage toward parasitism.
Phenomena of true symbiosis occur between plants and plants, between animals and animals, or between plants and animals. From cases of ectosymbiosis, in which the associated species retain a certain autonomy, one arrives at cases of endosymbiosis, characterized by a notable interpenetration of the symbionts into a single complex (Caullery).
One of the most interesting symbioses among plants is that found in lichens, of which every species (as Schwendener discovered in 1867) results from the intimate association of a fungus with a unicellular alga. The advantage of this coexistence (lichen consortium) derives from the fact that the alga, being autotrophic, synthesizes carbohydrates, while the fungus, being heterotrophic and capable of absorbing water and mineral substances from the environment, uses them to manufacture with them quaternary substances (fats, proteins). Another important plant symbiosis is that which occurs between legumes and Bacterium radicicola. The latter, localized in the roots of the plant where it generates characteristic tubercles, has the power to assimilate nitrogen of atmospheric origin, generating nitrogenous compounds from it, which are then used by the legume. A third example, very widespread, is that of mycorrhizae, or associations of fungi with herbaceous or arboreal plants (Kaminski, 1881; Frank, 1885). The mycelia of the fungi, wrapping around or interpenetrating the roots of the plants and acting almost as root hairs, draw water and mineral salts from the soil and supply them to the plant, while the plant supplies the fungus with the carbohydrates synthesized through chlorophyll activity by the green subaerial apparatuses. The partial digestion of fungal hyphae by the plant could also provide the latter with a food rich in nitrogen.
In symbioses between animals, the advantages obtained by the symbionts generally consist in defense against predatory organisms, or in the greater ease of procuring food. A well-known example is that of a group of soft-abdomen crustaceans (hermit crabs), which find protection by living inside empty shells of gastropod mollusks and which often associate in symbiosis with coelenterates (sea anemones) well provided with stinging cells, fixed on the shell of the mollusk. The stinging batteries of the sea anemones constitute a weapon of defense for the hermit crab; on the other hand the sea anemones derive advantage from the food leftovers of the hermit crab and, above all, from the possibility of being carried passively during the wanderings of the latter, thus becoming vagrant, from the sedentary state they would normally have had. Analogous symbioses, with similar reciprocal advantages, are established in the sponges Suberites, which implant themselves on the exoskeleton of crabs of the genus Dromia; or also in certain small fish, which are accustomed to living among the tentacles of sea anemones or jellyfish, thus defending themselves from the assault of their own predators, and at the same time favoring, through their movements, the arrival of food particles suspended in the water to the oral opening of the coelenterate.
A separate mention must be devoted to symbioses between protozoa and metazoa. One cites the case of termites, whose intestine hosts a considerable number of flagellates, indispensable for the digestion of the wood on which these insects feed; and that of ruminants, whose stomach harbors certain species of ciliates, necessary for the digestion of the cellulose of the membrane of plant cells.
Numerous and important, finally, are the symbioses between animal and plant organisms (Buchner). The symbiotic plant, in these cases, is generally a microscopic form belonging to the groups of fungi, algae or bacteria. Among the most widespread examples, one may recall that of various protozoa or metazoa (sponges, coelenterates, annelids, mollusks, etc.), whose cells present a yellow or green coloration due to the presence, in the cytoplasm, of unicellular algae (zooxanthellae, zoochlorellae), which take advantage of the protection offered by the animal and use, for the chlorophyll function, the carbon dioxide eliminated by it, while in turn the animal, at least up to a certain point, can exploit both the oxygen and the starch produced by the alga.
More complex are the cases designated by Pierantoni with the name of hereditary physiological symbiosis. These are mutualistic associations, constant and indispensable, in which a lower plant (blastomycete or bacterium) lives associated with an animal that, by various mechanisms, transmits the symbiont to its own descendants. In many insects such a symbiosis has nutritive functions. Thus the female of Icerya purchasi presents at the sides of the intestine a special tissue (mycetome) stuffed with blastomycetes, which find their nourishment in the animal cells and digest to the advantage of the insect the carbohydrates on which the latter feeds. The transmission of the blastomycetes to subsequent generations is ensured by their passage from the mycetome to the oocytes contained in the ovaries. Analogous facts have been found in aphids. In certain cases, the production of vitamins indispensable to the host by the symbiotic plant appears demonstrated.
Other cases of physiological symbiosis, between animals and bacteria, appear connected with phenomena of bioluminescence (Pierantoni, Buchner). Thus in the luminous organs of various insects the presence of bacteria has been noted, which participate more or less directly in the biochemical processes of light production. Very numerous photogenic bacteria have also been found within the luminous organs of marine animals (cephalopods, tunicates, fish). Not all cases of bioluminescence, especially frequent in abyssal faunas, however, seem attributable to phenomena of symbiosis with bacteria.
Bibliography4 works
Enrico Vannini