Symbiosis

SYMBIOSIS (from the Greek syn—with, together, and bios—life), a form of mutual coexistence of two organisms (or organic groups) associated with obtaining mutual benefit. The term symbiosis, introduced in 1867 by de Bary, has various interpretations and, in the broadest sense, is understood as the coexistence of organisms, regardless of its character and the consequences resulting from it for both participants. From this point of view, parasitism represents nothing other than a form of symbiosis. Even if one does not adhere to this extreme point of view, one must admit that the concept of symbiosis is somewhat vague, and a certain arbitrariness is possible in the interpretation of the phenomena observed here. Symbiosis can be observed in both plants and animals, and it is encountered in exactly the same way among microorganisms. In the latter, phenomena of changes in properties—pathogenicity, virulence, etc.—have been described as a result of the symbiosis of two species of microbes (see Paraimmunity). Phenomena of symbiosis in microorganisms, which are still comparatively little studied, represent one of the most important problems of modern microbiology. The normal flora of humans (oral cavity, intestine), which was usually considered

indifferent, acquires in the light of new research (L. G. Peretz) the significance of an important physiological factor; the disturbance of this symbiosis leads to the prevalence of pathogenic microbes and to subsequent disease. Bacillus carriage (see) is also a special case of symbiosis. In various cases, the symbiosis of pathogenic microbes can lead to an intensification of their toxic properties (diphtheria bacillus and streptococci) or, conversely, they can mutually inhibit the growth and reproduction of each other (antibiosis). As an example of the symbiosis of microorganisms with plants, one can point to the symbiosis of the bacteria Bact. radicicola on the lateral roots of leguminous plants (see Nitrogen-fixing bacteria). For cycads, special coral-like organs have been described, arising as a result of the accumulation of the same Bact. radic. and the alga Anabaenae, which gives these formations a green color. On the leaves of the tropical plant

Pavetta, there are nodules formed by accumulations of nitrogen-fixing bacteria.

As an example of symbiosis among plants, one should first of all mention lichens, which, since the works of Famintsyn and Baranetsky, and especially after the research of Schwendener, are considered a complex organism—a symbiosis of a fungus with an alga. Experimentally, both components (especially the fungus) can be separated and cultivated separately. The reproduction of lichens occurs with the help of special formations—soredia, where both fungal hyphae and algal cells are present. The meaning of symbiosis in lichens is not entirely clear; besides the usual interpretation of the coexistence of a fungus and an alga as mutually beneficial, there are views that consider this phenomenon as parasitism (the "exploitation") of one or the other component. The character of mutual benefit for the symbionts is apparently connected with the difference in the methods of nutrition of the green alga and the fungus. It is necessary to note a new property of lichens compared to the original forms—their extraordinary hardiness. In 1885, Frank introduced the concept of mycorrhiza—the symbiosis of a fungus with the roots of certain higher plants (heather, orchids, oak, etc.). In the absence of mycorrhiza, the plant (heather) remains underdeveloped, devoid of roots (Fig. 1). A distinction is made between ectotrophic mycorrhiza, when the threads of the fungus envelop the plant (root) from the outside, and endotrophic, when the hyphae penetrate into the cells and intercellular spaces of the symbiont plant. The symbiosis of oak roots with truffles is well known (Fig. 2). The significance of mycorrhiza is not entirely clear; apparently, it is a matter of increased absorption of salts from the soil with the help of the numerous branches of the fungus; some assume that symbiosis with the fungus creates the ability to assimilate atmospheric nitrogen and makes it possible for plants to exist on nitrogen-poor soil. A very vivid example of symbiosis is presented by the endotrophic mycorrhizae of orchids. Orchid seeds are usually only viable if they are infected by a fungus. This infection causes the formation of new organs—tubers, the underground root of the orchid, the "bird's-nest orchid," etc. A number of data show that for the flowering of orchids, the presence of stem mycorrhiza is necessary. The significance of symbiosis for the flowering plant is apparently reduced to the presence of a reserve of protein substances (part of the hyphae is digested), and for the fungus—to the supply of water and salts entering the plant. Figure 1. Heather of the same age. On the left—a sterile plant (without roots), on the right—"infected" with the symbiotic fungus.

Fig. 2. Symbiosis of oak roots with truffles.

Standing somewhat apart are the widely known cases of a peculiar symbiosis of animals with plants, in which the former assist in the cross-pollination of the latter (the moth Pronuba in the pollination of the yucca and many other examples). Pollination is carried out thanks to the sequential visiting of male and female flowers by animals and the transfer of pollen to the female organs of the plants. All these cases should be considered as symbioparasitism, since the animals usually cause some harm to the plant by eating part of the ovules, castrating part of the flowers, etc. Some cases of indirect symbiosis can be seen in the role that animals play in the dispersal of fruits and seeds; the latter are equipped with special adaptations of the hook or barb type, etc., which make it possible to attach to the body of the animals eating the plants and move with them over a long distance. Examples of the symbiosis of animals with plants are very numerous; here, first of all, one should point to unicellular green and yellow algae (zoochlorellae and zooxanthellae), found in the bodies of various representatives of animals. This type of symbiosis has been established for amoebae (Amoeba pelomyxa), radiolarians, sponges, worms, polyps, insects, up to highly organized animals. The significance of symbiosis is connected with the possibility for the animal symbiont to synthesize organic substances, and for the plant—with feeding on the food residues of the animal. Green algae isolated from the body (previously considered as animal chlorophyll) are capable of independent existence on a nutrient medium. "Sponge-algae," which are an alga covered by a sponge, are extremely common. Such cases are especially frequent among sponges of the species Reniera, Spongia, etc. Symbiosis is apparently hereditary, i.e., the eggs of sponges contain spores of algae. One can also mention the case of the symbiosis of the green alga Oedogonium on the surface of the body of the dragonfly Aeschna. The symbiosis of animals with microorganisms—bacteria and yeast fungi—is extremely widespread. Especially in insects, a whole series of special organs has been noted, filled with microorganisms and obviously playing a significant role in the physiology of these animals, mainly in digestive processes. These include the so-called bacteroid organs of some worms, symbiotic colonies of microorganisms in the cells of the intestine of the ant Camponotus, the brush-like organ and vaginal glands of some beetles, the so-called false yolk of aphids of the genus Aphidae, the abdominal organ of cicadas, etc. Microbes are especially common in the organs of blood-sucking animals (digestive glands of ticks, esophageal glands of leeches, etc.) (Fig. 3). The "fat body" of butterflies, which represents a "symbio-organ," deserves special mention; the activity of the symbionts in this case apparently determines the maturation of the sex elements (Pospelov). Symbiotic accumulations of microorganisms have been described for many mollusks and tunicates (Bojanus organ). It is very important to note that the widespread phenomenon of biological luminescence (see Luminescence) in many cases (in insects, cephalopods, tunicates—Pyrosoma) is caused by the symbiosis of these animals with luminous bacteria (Pierantoni). Somewhat less frequently, cases of the symbiosis of animals with animals have been described. The most famous are the repeatedly cited cases of the symbiosis of the crab

Fig. 4. The sea anemone Sagartia parasitica on a shell inhabited by a hermit crab.

the hermit crab Eupagurus with the sea anemone Adamsia (mutual benefit: for the sea anemone—locomotion, for the crab, which is unprotected by a shell—protection of the body with the help of the stinging organs of the sea anemone) (Fig. 4); in the same way, one can mention the symbiosis of the crab Mellia with a sea anemone, the female lobster with the leech Mistrobdella (consumption of dying eggs on the abdomen of the lobster), etc. There exists a school of thought (Buchner, Portier, Famintsyn, Merezhkovsky, Kozo-Polyansky, and others) that significantly expands the concept of symbiosis and puts forward the idea of s y m b i o g e n e s i s as the most important moment of biological evolution. From this point of view, all organisms (except bacteria) represent consortia—biological communities existing in symbiosis; evolution is possible thanks to the complication of the consortium, the addition of new members to it, similar to how a new type of lichen arises from the symbiosis of fungi and algae. Using the rich factual material accumulated by biology in the field of symbiosis, the authors go quite far, asserting, for example, that cellular organelles, up to the nucleus, chromosomes, chondriosomes, etc., represent symbiotic microorganisms; in the same way, many organelles of plant cells—plastids (chloroplasts), aleurone grains, etc.—are considered as the result of the symbiosis of microorganisms with plant cells. The authors also include here grafts in higher plants, heterotransplantations in animals, etc. These extreme views find some confirmation in the works of a number of authors (Portier, V. N. Lyubimenko), who cultivated: the former—chondriosomes, the latter—chlorophyll grains outside the organism, and established their ability to survive outside the cell. While one cannot deny some significance of symbiosis as a factor creating new biological individualities (lichens), the views of the representatives of the theory of symbiogenesis still seem one-sided and are difficult to reconcile with modern cytological views. In any case, they require a series of verification experiments. The very concept of symbiosis, as we have seen, is extremely vague and easily opens the way to anthropomorphic and idealistic interpretations.