Symbiosis in the plant world. Symbiosis: examples in nature

Photo of symbiosis of mushrooms with roots

A striking example of fungal symbiosis is mycorrhiza - a community of fungi and higher plants (various trees). With such “cooperation” both the tree and the mushroom benefit. Settling on the roots of a tree, the fungus performs the function of absorbing root hairs and helps the tree absorb nutrients from the soil. With this symbiosis, the fungus receives ready-made organic substances (sugars) from the tree, which are synthesized in the leaves of the plant with the help of chlorophyll.

In addition, during the symbiosis of fungi and plants, the mycelium produces substances such as antibiotics that protect the tree from various pathogenic bacteria and pathogenic fungi, as well as growth stimulants such as gibberellin. It has been noted that trees under which cap mushrooms grow practically do not get sick. In addition, the tree and the mushroom actively exchange vitamins (mainly groups B and PP).

Many cap mushrooms form symbiosis with the roots of various plant species. Moreover, it has been established that each type of tree is capable of forming mycorrhiza not with one type of fungus, but with dozens of different species.

In the photo Lichen

Another example of the symbiosis of lower fungi with organisms of other species is lichens, which are a union of fungi (mainly ascomycetes) with microscopic algae. What is the symbiosis of fungi and algae, and how does such “cooperation” occur?

Until the middle of the 19th century, it was believed that lichens were separate organisms, but in 1867, Russian botanists A. S. Famintsyn and O. V. Baranetsky established that lichens are not separate organisms, but a community of fungi and algae. Both symbionts benefit from this union. Algae, with the help of chlorophyll, synthesize organic substances (sugars), which the mycelium feeds on, and the mycelium supplies the algae with water and minerals, which it sucks from the substrate, and also protects them from drying out.

Thanks to the symbiosis of fungus and algae, lichens live in places where neither fungi nor algae can exist separately. They inhabit hot deserts, high mountains and harsh northern regions.

Lichens are even more mysterious creatures of nature than mushrooms. They change all the functions that are inherent in separately living fungi and algae. All vital processes in them proceed very slowly, they grow slowly (from 0.0004 to several mm per year), and also age slowly. These unusual creatures are distinguished by a very long life expectancy - scientists suggest that the age of one of the lichens in Antarctica exceeds 10 thousand years, and the age of the most common lichens that are found everywhere is at least 50-100 years.

Thanks to the collaboration of fungi and algae, lichens are much more resilient than mosses. They can live on substrates on which no other organism on our planet can exist. They are found on stone, metal, bones, glass and many other substrates.

Lichens still continue to amaze scientists. They contain substances that no longer exist in nature and which became known to people only thanks to lichens (some organic acids and alcohols, carbohydrates, antibiotics, etc.). The composition of lichens, formed by the symbiosis of fungi and algae, also includes tannins, pectins, amino acids, enzymes, vitamins and many other compounds. They accumulate various metals. Of the more than 300 compounds contained in lichens, at least 80 of them are found nowhere else in the living world of the Earth. Every year, scientists find in them more and more new substances that are not found in any other living organisms. Currently, more than 20 thousand species of lichens are already known, and every year scientists discover several dozen more new species of these organisms.

From this example it is clear that symbiosis is not always simple cohabitation, and sometimes gives rise to new properties that none of the symbionts had individually.

There are a great many such symbioses in nature. With such a partnership, both symbionts win.

It has been established that the desire for unification is most developed in mushrooms.

Mushrooms also enter into symbiosis with insects. An interesting association is the connection between some types of molds and leaf-cutter ants. These ants specifically breed mushrooms in their homes. In separate chambers of the anthill, these insects create entire plantations of these mushrooms. They specially prepare the soil on this plantation: they bring in pieces of leaves, crush them, “fertilize” them with their feces and the feces of caterpillars, which they specially keep in the neighboring chambers of the anthill, and only then introduce the smallest fungal hyphae into this substrate. It has been established that ants breed only mushrooms of certain genera and species that are not found anywhere in nature except anthills (mainly fungi of the genera Fusarium and Hypomyces), and each species of ants breeds certain types of mushrooms.

Ants not only create a mushroom plantation, but also actively care for it: they fertilize, prune and weed. They cut off the emerging fruiting bodies, preventing them from developing. In addition, ants bite off the ends of fungal hyphae, as a result of which proteins accumulate at the ends of the bitten off hyphae, forming nodules resembling fruiting bodies, which the ants then feed on and feed their babies. In addition, when the hyphae are trimmed, the mycelium of the fungi begins to grow faster.

“Weeding” is as follows: if mushrooms of other species appear on the plantation, the ants immediately remove them.

It is interesting that when creating a new anthill, the future queen, after the nuptial flight, flies to a new place, begins to dig tunnels for the home of her future family, and creates a mushroom plantation in one of the chambers. She takes mushroom hyphae from an old anthill before flight, placing them in a special suboral pouch.

Termites are also bred in similar plantations. In addition to ants and termites, bark beetles, boring insects, some types of flies and wasps, and even mosquitoes are involved in “mushroom farming.”

German scientist Fritz Schaudin discovered an interesting symbiosis of our ordinary blood-sucking mosquitoes with actinomycetes yeast fungi, which help them in the process of sucking blood.

DETERMINING THE PROBLEM OF THE LESSON

Antoshka: On the bark of trees and stones I saw plants in the form of thin leathery, crumpled plates and gray branched tubes. Biologist: These are not plants, but lichens - a special group of living organisms. They are more like an entire ecosystem than an individual organism.

Formulate the questions you need to ask the biologist in order to understand his words. Compare with the author's version (p. 171).

How do lichens differ from plants and fungi?

LET'S REMEMBER WHAT WE KNOW

What is symbiosis? (§ 13)

Symbiosis is the mutually beneficial cohabitation of organisms of different species.

What is an ecosystem? (§2)

An ecosystem is a unity of inanimate nature and living organisms of different “professions”.

What examples of symbiosis have you already studied? (§ 13, 17)

Symbiosis of nodule bacteria with leguminous plants; cows with bacteria in their stomach; mushrooms with trees and herbs.

WE SOLVE THE PROBLEM, DISCOVER NEW KNOWLEDGE

Find answers to the questions in the text:

1) Why can’t lichens be called plants?

2) What are the differences between this group and other organisms?

Lichens are a symbiosis of fungus and algae. Therefore, a lichen is not only a separate organism, but also an entire miniature “ecosystem” that can live independently.

Lichens differ significantly from other groups of organisms, including free-living fungi and algae, in their special biology: methods of reproduction, slow growth, attitude to environmental conditions, etc.

Lichens often live in places where other land plants cannot survive.

Make a guess as to what the text with this title says. What is the reason for this feature of lichens?

The text explains how lichens have the advantage of surviving in conditions unfavorable to other organisms.

One lichen organism already contains both producer algae and consumer fungi. Therefore, a lichen is not only a separate organism, but also an entire miniature “ecosystem” that can live independently. With the symbiosis of a fungus and algae, it is possible to colonize places where they are not viable without each other.

To check your assumption, read the text, conducting a dialogue with the author: B - ask a question to the author of the text; O - predict the answer; P - check yourself in the text. After reading the text, draw a conclusion about the lesson problem.

Which “professions” exactly and why? O Try to remember.

One lichen organism already contains both producer algae and consumer fungi.

Only through joint efforts can they maintain the circulation of substances.

Conclusion: The symbiosis of fungus and algae in lichen allows them to survive in conditions unfavorable for other organisms.

What properties should the upper surface of a lichen have?

The upper surface of the lichen should be dense and smooth.

APPLYING NEW KNOWLEDGE

1. What are lichens?

Lichens are not plants, but a symbiosis of fungus and algae.

2. What groups of lichens do you know?

1. Scale lichens are thin films of different colors that adhere tightly to the surface on which they live.

2. Foliaceous lichens in the form of plates, in some places tightly pressed to the ground, and in others extending from it.

3. Bushy lichens in the form of funnels, branching tubes, branched ribbons and cords.

3. Why can lichens settle in the driest places?

Lichen becomes saturated with moisture after rain or dew.

4. How do fungi and algae, coexisting in a lichen, help each other?

In a lichen, the fungus covers the algae and retains moisture for it, and the algae supplies the fungus with organic substances.

5. Why are lichens considered a separate group of living organisms, and not an ecosystem of co-living algae and fungi?

The fungus and algae in the lichen interact very closely with each other.

The types of fungi that make up a lichen do not exist in nature without algae, which is why lichens cannot be an ecosystem of algae and fungi living together.

6. Imagine a biosphere where only lichens grow. What problems would its inhabitants face? Have one of you suggest ideas and the other evaluate. Then switch tasks.

One of the problems that a biosphere consisting of lichens alone would face is the accumulation of decay products of these organisms due to the absence of destroyers. The circulation of substances would cease, the planet would turn into a dump of dead lichens.

Another problem could be the depletion of carbon dioxide in the atmosphere. Due to the process of photosynthesis occurring in algae, oxygen would actively accumulate. Of course, it is partially used in the respiration of algae and lichen fungi, but this volume may not be enough to maintain the balance of oxygen and carbon dioxide.

7. Why are there no lichens in the shape of a tall tree?

Lichens grow very slowly: over the course of a year they increase by a few millimeters, and some by a fraction of a millimeter.

MY BIOLOGICAL RESEARCH

Moisten foliose or fruticose lichen. Examine the ground side of a leafy plant or the inner side of a bushy plant under a microscope. Look at the top side. Examine a section of lichen. Try to find algae cells and fungal hyphae. Sketch them.

Lichens (lichenized fungi) are complex organisms, the body (thallus) of which constantly consists of two components - a fungus (mycobiont) and an algae or cyanobacteria (photobiont), forming a symbiosis characterized by special morphological and anatomical types, as well as unique physiological and biochemical processes. The dual nature of lichens was discovered in the 60s of the 19th century by the scientist S. Schwendener. The cohabitation of photobiont and mycobiont in lichen is mutually beneficial. The photobiont supplies the mycobiont with carbohydrates, receives water and minerals, and protection from drying out and solar radiation.

The photobionts of most lichens belong to green algae from 34 genera, in a minority - to cynobacteria (blue-green algae) from 10 genera, as well as yellow-green algae from 1 genus Heterococcus, and brown algae from 1 genus Petroderma.

Of the green algae, the photobionts most often present are algae from the genus Trebouxia (in 70% of lichens), which is practically never found in a free state (according to other sources, it is very rarely found in a free state); from other genera of algae, free-living algae of the genus Trentepohlia are often represented and Cladophora.

Among blue-green algae, cyanobacteria of the genera Nostoc and Anabaena are often present as photobionts in lichens.

Located in the lichen thallus, the photobiont usually exists in the form of individual cells or short filaments, losing the ability to reproduce by zoospores and sexually. It reproduces only by division in two or by autospores.

A lichen may contain more than two partners. Quite often, three-component lichens have one mycobiont and two photobionts (green algae - primary) and cyanobacteria (blue-green algae), which is localized in special structures - cephalodia .

For example, lichen Peltigera aphthosa has a photobiont in the thallus, a unicellular green algae from the genus Coccomyxa, and on the surface of the thallus cephalodia develop in the form of small tubercles, plates or scales with cyanobacteria from the genus Nostoc.

Mycobionts of lichens belong mainly to marsupial fungi (usually discomycetes, less often pyrenomycetes and loculoascomycetes). Such lichens are called marsupial lichens, for example, with perithecia - Dermatocarpon sp. (foliate epilithic), Verrucaria sp. (scale epilithic), with apothecia – species of the genera Xanthoria, Peltigera, Parmelia.

In marsupial lichens, specific types of thalli (scaleous, leafy, bushy) and specific lichen substances were formed (75 compounds were identified only in lichens). These are aromatic substances - compounds formed by a combination of phenolic units: quinones, triterpenoids, phenolcarboxylic acids (usnic, evernic), depsides, xanthones, etc.).

Only in a few dozen species, mainly tropical lichens, the mycobiont belongs to basidiomycetes (aphyllophoroid and agaricoid hymenomycetes). Such lichens are called basidial. In this case, the symbiosis of the fungus and algae does not lead to the formation of new types of thalli, as in marsupial lichens, or to the formation of specific lichen substances. The fungus and algae of such lichens can exist separately. Basidial lichens follow the shape of the fruiting bodies of free-living basidiomycetes and contain a layer of algae.

For example, basidiomycete Cora pavonia(mycobiont – Thelephora sp., photobiont – green algae Chlorococcum sp.) grows on the soil and looks like large (up to 10 cm in diameter) light gray plates bearing hymenia on the underside. Other basidial lichen Multiclavula mucida(mycobiont – Clavulinopsis sp., photobiont – green algae – Coccomyxa sp.) forms low, club-shaped fruiting bodies similar to the horned fungus, which grow from a fungal-algal film attached to rotten wood. Basidial lichen Omphalina ericetorum has a fruiting body in the form of a cap mushroom.

Sometimes a specific complex symbiotic formation from fungal hyphae is also considered a lichen Geosiphon pyriforme(division Zygomycota), living on soil and in water. Inside the hyphae are threads of blue-green algae (cyanobacteria) from the genus Nostoc.

Fruiting bodies are not always found in lichen mycobionts. In some, they may never form at all, so there is a group of “imperfect lichens,” of which over 100 forms have been described on different substrates. These are sterile thalli that reproduce only through soredia. Of the “imperfect lichens,” the most common are lepraria (genus Lepraria). They form powdery (powdery) deposits on various substrates: stones, rocks, tree trunks, mosses. Typically these plaques are whitish-gray ( L. aeruginosa), sometimes greenish-yellow ( L.chlorina) or golden yellow ( L.candelaria).

There are three main concepts of the essence of lichen symbiosis:

2) the lichen association is mutually beneficial, mutualistic (A. de Bary);

3) lichen is a single independent organism (J. Reinke, B. M. Kozo-Polyansky). However, the current level of knowledge does not provide any basis for the assertion that relationships between genetically different organisms can lead to the emergence of a new independent organism of a special systematic category.

According to the first two concepts, lichen symbiosis is considered as one of the types of biotic relationships of organisms, that is, as an association of two genetically isolated organisms, which is based on trophic connections.

1) the use of photobiont photosynthesis products by the fungus, which is accompanied by the movement of 40% or more (60%) of the carbon fixed during photosynthesis;

2) absorption of nitrogen fixed by lichen cyanobacteria, also mainly by the fungal component, leaving only 3% of the nitrogen for the photobiont;

3) the presence of absorption hyphae in the mycobiont, penetrating into the photobiont cell directly to the protoplast (intracellular haustoria), or penetrating or pressing against its shell (intrathecal haustoria) and serving to transfer nutrients from the photobiont to the mycobiont. Most often, the fungus receives the polyhydric alcohol ribitol from the photobiont, and less often glucose.

Symbiosis of fungi and animals. There is a known symbiosis of fungi and social insects - tropical ants and termites, which grow mushrooms in their nests and feed on them. Hymenomycetes from the genus Rozites live in symbiosis with ants, but in anthills they are represented only by mycelium. The mycelium is constantly nibbled and a head of swollen cells forms on its short branches, which the ants feed on. The female, flying off on a mating flight, takes with her part of the mycelium.

Other examples of symbiosis between fungi and animals include associations of yeast with xylophages, endosymbiotic groups of yeast in the intestines of diplopods, complex zoomicrobial complexes formed in the rotting tissues of some cacti, in fermenting spring sap of trees, populations of debariomycetes in the nests of forest ants, cohabitation of septobasidiomycetes and scale insects. .

Symbiosis of fungi with higher plants. An example of such a symbiosis is mycorrhiza – cohabitation of fungal hyphae with the roots of higher plants. It is formed in most plants, with the exception of aquatic ones. The higher plant provides the fungus with organic substances, and the fungus supplies the plant with phosphorus, nitrogen, and mineral nutrition elements. Without a higher plant, the mushroom does not form fruiting bodies.

The process of absorption of minerals by mushrooms is carried out in two ways:

1) by increasing the contact zone between root cells and soil,

2) the fungus is capable of converting previously insoluble or organic forms of phosphates into soluble forms, which are inaccessible for absorption by non-mycorrhizal plants.

Mycorrhiza for a plant is multifunctional. Mycorrhiza increases the permeability of mesophyll with carbon dioxide, increases the concentration of chlorophyll in the leaves and stimulates photosynthesis, improves the water regime of plants, reduces the entry of heavy metals into the shoots of plants growing on soils with a high content of readily available metals, helps to increase the resistance of plants to salinity on alkaline soils, affects soil structure and population dynamics of soil organisms.

Mycorrhizal fungi are a special ecological group of fungi. The mycelium of mycorrhizal fungi is concentrated, as a rule, in the epiblema and mesoderm of the roots and is not found in the endoderm, central cylinder, and meristem of the root apex.

There are ectotrophic, endotrophic and ecto-endotrophic mycorrhizae (Fig. 4-6).

At ectotrophic In mycorrhizae, the hyphae of the fungus entwine the tip of the root, forming an outer sheath with hyphae extending into the soil, replacing the root hairs of the plant. In this case, the root does not have its own root hairs. Characteristic of woody plants. The main part of ectomycorrhiza-formers are basidiomycetes.

At endotrophic mycorrhiza, the hyphae of the fungus penetrate into the root tissues (through the intercellular spaces and intracellularly) and only slightly come out (the root bears normal root hairs). When the fungus grows inside the root, tangles of hyphae often form - vesicles and intracellular branches in the form of haustoria - arbuscules . This type of mycorrhiza is called arbuscular mycorrhiza .

Endotrophic mycorrhiza is characteristic of most herbaceous plants, primarily orchids. It is formed mainly by microscopic fungi from 120 species with non-cellular mycelium from the division Zygomycota (pp. Glomus (40 species), Acaulospora, Gigaspora, Sclerocystis, etc.), or fungi with cellular mycelium from the divisions Ascomycota and Deuteromycota (p. Rhizoctonia). For most orchid species, such mycorrhiza is obligate, but for other herbaceous plants it is not so obligatory.

Transitional type – ecto-endotrophic mycorrhiza. In this case, the hyphae densely entwine the root from the outside and give off abundant branches that penetrate into the root. Along the intercellular spaces and intracellularly, forming vesicles and arbuscules in cells. In addition, the hyphae of the fungus, passing between the cells of the rhizoderm, form a single-layer plexus - the “Hartig network”, which, perhaps, has the significance for fungi of the mechanism of their genotypic variability. The network has a coenocytic nature. The abundance of nuclei freely located in the common cytoplasm creates opportunities for the parasexual process. In addition, the advantage of such a network organization for the exchange of nutrients between the fungus and the plant, as well as for the accelerated movement of substances within the hyphae, is obvious.

The outer free hyphae of the fungus disperse widely in the soil from the root, replacing root hairs. Most trees. This mycorrhiza is formed mainly by macromycetes from the Basidiomycota department of the hymenomycetes group (cap mushrooms). Among marsupials – species of the genus Tuber and Elaphomyces, which enter into symbiosis with beech and oak. For most mycorrhizal fungi, this symbiosis is obligatory.

All components of the animal and plant world are closely interconnected and enter into complex relationships. Some are beneficial for the participants or even vitally important, for example lichens (the result of a symbiosis of a fungus and algae), others are indifferent, and still others are harmful. Based on this, it is customary to distinguish three types of relationships between organisms - neutralism, antibiosis and symbiosis. The first one, in fact, is nothing special. These are relationships between populations living in the same territory in which they do not influence each other and do not interact. But antibiosis and symbiosis are examples that occur very often; they are important components of natural selection and participate in the divergence of species. Let's look at them in more detail.

Symbiosis: what is it?

It is a fairly common form of mutually beneficial cohabitation of organisms, in which the existence of one partner is impossible without the other. The most famous case is the symbiosis of a fungus and algae (lichens). Moreover, the first receives photosynthetic products synthesized by the second. And the algae extracts mineral salts and water from the hyphae of the fungus. Living separately is impossible.

Commensalism

Commensalism is actually the unilateral use of one species by another, without exerting a harmful effect on it. It can come in several forms, but there are two main ones:

All others are to some extent modifications of these two forms. For example, entoikia, in which one species lives in the body of another. This is observed in carp fish, which use the cloaca of holothurians (a species of echinoderm) as a home, but feed outside it on various small crustaceans. Or epibiosis (some species live on the surface of others). In particular, barnacles feel good on humpback whales, without disturbing them at all.

Cooperation: description and examples

Cooperation is a form of relationship in which organisms can live separately, but sometimes unite for common benefit. It turns out that this is an optional symbiosis. Examples:

Mutual cooperation and cohabitation in the animal environment are not uncommon. Here are just some of the most interesting examples.

Symbiotic relationship between plants

Plant symbiosis is very common, and if you look closely at the world around us, you can see it with the naked eye.

Symbiosis (examples) of animals and plants

Examples are very numerous, and many relationships between different elements of the plant and animal world are still poorly understood.

What is antibiosis?

Symbiosis, examples of which are found at almost every step, including in human life, as part of natural selection, is an important component of evolution as a whole.

Symbiosis, or the cohabitation of two organisms, is one of the most interesting and still largely mysterious phenomena in biology, although the study of this issue has a history of almost a century. The phenomenon of symbiosis was first discovered by the Swiss scientist Schwendener in 1877 while studying lichens, which, as it turned out, are complex organisms consisting of an algae and a fungus. The term “symbiosis” appeared in scientific literature later. It was proposed in 1879 by De Bary.

Among the symbioses, symbioses involving algae occupy not the least place. Algae are capable of entering into symbiotic relationships not only with each other, but also with representatives of various systematic groups of organisms of both the animal and plant kingdoms (bacteria, unicellular and multicellular animals, fungi, mosses, ferns, gymnosperms and angiosperms). However, the list of such algae is very limited. Of the vast group of blue-green algae, symbiosis with fungi (lichen symbiosis) can be established by representatives of no more than 5-7 genera, of which Nostoc, Gloeocapsa, Scytonema and Stigonema are the most common.

Analysis of various symbioses revealed the extremely diverse nature of the relationships between partners, the varying degrees of their influence on each other. One of the simplest cases is the settlement of some organisms on the surface of others.

As is known, plants that live on other organisms but feed on their own are called epiphytes. Epiphytes also include a large group of algae. Algae especially often epiphyte on underwater plants and waterfowl, sometimes covering them with a dense coating (Fig. 46). During epiphytation, very fragile and short-term relationships are established between the participants, which, however, can already be considered as symbiotic. Since the epiphytic algae and the host have a rather weak influence on each other, epiphytism in algae is considered to be the most primitive form of symbiosis. He is even classified as “indifferent.” It is difficult to completely agree with such a statement. Epiphytes really do not cause direct harm to the organism to which they are attached, but indirect damage is still caused. It is well known, for example, that the legs of waterfowl mites, spiders and beetles overgrown with algae become less mobile, and the plants are strongly shaded by the epiphytes that have settled on them and find themselves in conditions unfavorable for photosynthesis. The phenomenon of fouling is often encountered when breeding aquarium plants, which can be severely inhibited by the algae living on them.

Unfortunately, the phenomenon of epiphytism from a biological point of view has been studied extremely poorly. It is possible that a much more complex relationship is established between the epiphyte and its host than we usually imagine.

In addition to surface attachment, algae can live in the tissues of other organisms, both extracellularly (in mucus, intercellular spaces, rarely in the membranes of dead cells - Fig. 47) and intracellularly (in the contents of living undamaged cells - Fig. 48). Based on their habitat, such algae belong to the group of plants endophytes.

,

Extracellular and especially intracellular zndophytes from among algae form more complex symbioses in comparison with zpiphytes - endosymbioses. They are characterized by the presence of more or less close, permanent and strong ties between partners. Zndosymbioses can be detected only with the help of special cytological studies.

The most numerous group consists of syndosymbioses of unicellular green and yellow-green algae with unicellular animals (Fig. 48, 1). These algae are called zoochlorella and zooxanthellae, respectively. Among multicellular animals, green and yellow-green algae form syndosymbioses with freshwater sponges, hydra, etc. (Fig. 48, 2). Blue-green algae form with protozoa and some other organisms a peculiar group of endosymbioses called syncyanosis; the resulting morphological complex of two organisms is called cyanoma, and blue-green algae in it - cyanella(Fig. 48, 3).

Comparison of various endosymbioses makes it possible to outline successive stages of complication of the morphological and functional subordination of partners. Thus, some syndosymbioses exist for a very short time and then disintegrate, which is evidence of their primitiveness. An example of this is the slimy colonial blue-green alga Woronichinia naegeliana. In almost 50% of cases, other blue-green algae (Lyngbya endophytica and Synechocystis endobiotica - Fig. 47.1) live in the mucus surrounding the spherical colonies of this algae. They reproduce intensively there, although they have an extremely pale, barely noticeable color. This is probably due to their ability to utilize ready-made organic compounds, which are formed in abundance during the breakdown of mucus.

Over time, the intensive growth of algae in the mucus of voronihinia leads first to the suppression of cells, and then to the disorganization and death of the entire colony, and, consequently, the symbiosis as a whole.

The question arises: how do algae penetrate the tissues and cells of other organisms? Some organisms have special adaptations for this. Thus, the small Azolla fern (Azolla) floating in water has special cavities with narrow outlet openings on the underside of the leaves through which mucus is released. In these cavities, regardless of the geographical point of the globe where azolla grows (in America, Asia, Africa or Australia), colonies of a strictly defined type of blue-green algae - Anabaena azollae - settle. Over time, the cavities close and the algae trapped there is completely isolated. Attempts to infect Azolla with representatives of other genera and even species of blue-green algae were unsuccessful. This indicates that in the process of the emergence of this symbiosis, a rather specific physiological interdependence is established between the participants. This conclusion is also confirmed by the fact that the nitrogenous compounds produced by Azolla are completely absorbed by the Anabena specimens that endosymbioticate here, as a result of which they no longer have the function of fixing atmospheric nitrogen, which is characteristic of free-living representatives of this blue-green alga. In turn, Anabena additionally supplies the host tissues with oxygen and other waste products.

Despite the specialization of physiological processes that exists in these symbionts, not one of them undergoes any significant changes in its organization.

However, this is not the case for all endosymbioses of this type. The endosymbiotic lifestyle of algae most often leads to a partial or complete reduction of their cell walls. For example, in the tissues of the sea sponge Aplysilla, individuals of blue-green algae from the genus Aphanocapsa, the reduction of the cell membrane is expressed in a decrease in its thickness. Due to this, the protective properties of the shell are reduced, but its permeability increases. The latter quality undoubtedly improves the conditions for the transport of substances between the cells of the sponge and the algae symbiotic there.

Endosymbioses belonging to the category extracellular, already form quite stable functional and morphological complexes. This trend is even stronger in intracellular endosymbioses. The mechanism by which algae penetrate into the cells of other organisms without damaging them or disrupting normal life functions remains unclear. Part of the prerequisites for the emergence of intracellular endosymbioses may lie in the preservation of the holozoic type of nutrition in the cells of some organisms. Of all the known types of nutrition, the holozoic type is considered one of the most ancient.

In organisms with a holozoic type of nutrition, the captured prey, which includes algae, enters directly into the cell and is digested there. However, individual captured individuals, probably due to a combination of favorable circumstances, sometimes manage not only to remain intact inside the host cells, but also to develop adaptations to new, unusual living conditions and begin to reproduce there. As a result, a new type of relationship is established between organisms - symbiotic. This is probably how specimens of the mobile unicellular algae Euglena (Euglena gracilis) penetrate the epithelial cells of the hindgut of the larvae of some dragonfly species. Euglena cells remain green there throughout the entire period of their life together. They, however, lose mobility, but at the same time they never encyst. Apparently, in the same way, individuals of the unicellular green alga Carteria settle in the epidermal cells of the ciliated convolute worm (Convoluta roscoffensis). As it turned out, carteria cells, under the influence of a symbiotic lifestyle, although they undergo very significant changes (the membrane is completely reduced, and the cells are surrounded only by a thin plasma membrane - the plasmalemma, the stigma disappears, the internal organization of flagella is simplified), but they do not stop photosynthesizing. In turn, the worm acquires the ability to feed on the waste products of the algae, which are produced during the process of photosynthesis. In particular, it can live for 4-5 weeks without receiving any food from the outside. However, when the process of photosynthesis stops (for example, if the experiment is carried out in the dark), both the algae and the worm die. Moreover, worm larvae, deprived of algae cells, are not able to lead an independent existence. Artificial infestation with algae fails.

Intracellular syndosymbioses are undoubtedly easier to establish with those organisms whose cells do not have a hard shell throughout the entire life cycle or at least at one of its stages. Penetration of the symbiont into cells with hard shells is possible only if they are partially or completely destroyed. The latter can occur under the influence of specific enzymes produced by an organism entering into a symbiotic relationship. The strict specialization of organisms entering into symbiosis observed in a number of cases is probably explained precisely by this circumstance. Unfortunately, all attempts to detect at least traces of this kind of enzymes have so far been unsuccessful.

Some intracellular syndosymbioses, as occurs in dragonfly larvae, periodically disintegrate and are renewed again; others are continuously supported from generation to generation, since in these cases strong and lasting connections are established between the participants. The last group of endosymbioses could obviously arise due to the loss of that phase in the life cycle of the host organism that was favorable for the penetration of the symbiont into its cells. From this moment, apparently, the close joint life of the two organisms begins. In such cases, the transition to a symbiotic mode of existence is inevitably accompanied by a number of adaptive changes in both organisms. Sometimes these changes are morphologically insignificant and the symbiont can be recognized (for example, the nostok of a geosiphon, Fig. 48.3), and sometimes they are so significant that the symbiotic algae cannot be identified with any of the free-living algae.

Thus, in the vacuoles of one of the species of ciliated paramecia (Paramecium bursaria) a single-celled green alga is invariably present. Based on its morphology and behavioral characteristics, it can only be conditionally classified as a protococcal algae of the genus Ghlorella. It has been established that algae cells divide independently of the division of paramecium. Each of the newly formed daughter cells (autospores) of the algae is immediately enclosed in a special vacuole and, in this form, is subsequently distributed among the daughter ciliates.

In some cases, such close interdependent relationships develop between symbionts that they can no longer live outside of symbiosis. Obviously, they irreversibly lose the ability to independently produce a number of substances that come in finished form from the algae that symbiote with them. The reality of such an assumption was fully confirmed in experiments with hydra, which, it turns out, receives maltose in the required quantity from the cell of a green algae symbiotic there, the systematic affiliation of which could not be accurately established.

Sometimes non-decomposing endosymbioses lead to the formation of such a complex, the symbiotic nature of which is revealed with great difficulty. This happened with two algae - cyanophora and glaucocystis.

In 1924, a new algae for science was described, called paradoxical cyanophora (Cyanophora paradoxa, tables 5, 7). Later, a detailed study of this organism showed that cyanophora is a symbiosis of the colorless unicellular algae cryptomonas (division Pyrrophyta) and the intracellular blue-green algae (cyanella) of the genus Chroococcus (division Cyanophyta) that settles in it. The cells of the latter, under the influence of a symbiotic lifestyle, are so modified that they lose their typical appearance. This is expressed mainly in a strong reduction of the cell membrane.

It decreases not only in thickness, but also in the number of layers it contains: instead of the four-layer structure usually characteristic of free-living blue-green algae, it becomes two-layer.

The cyanelles that make up glaucocystis (Glaucocystis nostochinearum), a very peculiar single-celled algae described at the end of the last century, undergo even greater transformations. For a long time it was not possible to accurately determine its systematic position. Based on its blue-green coloration, it was first assigned to the division Cyanophyta. Subsequently, the identification of a number of characteristics completely unusual for blue-green algae (the presence of a morphologically formed nucleus, colored bodies, reproduction through autospores) made it possible to classify this organism as a green algae (division Chlorophyta). Only in the 30s of the current century was it finally established that glaucocystis is an extremely peculiar form of endosymbiosis of a discolored unicellular algae close to the genus Oocystis (Oocystis) and a rod-shaped blue-green algae, which has undergone such strong transformations here that it cannot be established exactly systematic affiliation is not possible. Equally, it can be any modified representative from a number of genera of unicellular rod-shaped blue-green algae. In symbioses of this kind, glaucocystis is so far the only example of establishing such a close relationship between partners. Blue-green algae (cyanella) are located in the cells of glaucocystis, either ordered in two groups, or randomly, randomly.

Cyanella and free-living blue-green algae are no different from each other in their fine organization. It is noteworthy that cyanella does not contain inclusions of reserve nutrients represented by various metabolic granules. Apparently, there is no need for this, since cyanella receive the substances they need directly from the host cell. At the same time, cyanella deliver to the host cells some products that they produce during photosynthesis. This is evidenced by the presence of starch grains in the cytoplasm of colorless cells of the host organism. This phenomenon is very unusual, since in all chlorophyll-bearing green plants the only place where starch grains are localized is the plastid (chloroplast). Under conditions of symbiosis, its participants probably achieve maximum specialization, due to which the symbiotic blue-green algae take on the functions of chloroplasts, but do not become them. The latter is supported by a significant difference in the organization of cyanella and plastids. The cells of the colorless symbiont glaucocystis lose the ability to independently form starch, which is formed there, obviously, with the direct participation of cyanella.

An electron microscope study of the cyanelles that make up Glaucocystis revealed a strong degree of cell wall reduction in them. It is preserved here in the form of a barely noticeable outline, which can only be detected if the quality of fixation and processing of the material is high. A more thorough study of the cyanelles showed that they are surrounded only by a thin (100 ± 10A) membrane called the plasmalemma. This degree of reduction of cell cover is a unique phenomenon among blue-green algae entering into symbiosis.

From the given characteristics of cyanella it is clear that they are nothing more than cells of blue-green algae, devoid of reserve substances and cell membranes.

The division of cyanella, like the cells of free-living blue-green algae, is carried out by constriction in half. It is autonomous and is not confined to the period of reproduction of the host cell. Each of its daughter cells usually contains several cyanellae. This ensures the continuity of the symbiosis. Unlike organelles, the distribution of cyanelles between the daughter cells of the host is random, so their number varies greatly. There is no doubt that the division itself and the nature of the divergence of cyanelles into daughter cells is regulated not by the host, which would be quite natural if they turned into organelles, but by the cyanelles themselves, which have retained all the properties of the cells. However, even in conditions of such a highly developed symbiosis, as exemplified by glaucocystis, both partners still retain their individual characteristics and autonomy. This is evidenced by their ability to exist separately outside the host cells. In a specially selected nutrient medium, isolated symbionts behave like independent organisms. They not only grow and develop successfully there, but also reproduce.

Among the symbioses formed with the participation of algae, the most interesting is the symbiosis of algae with fungi, known as lichen symbiosis. As a result of this symbiosis, a peculiar group of plant organisms emerged, called lichens. You can learn more about them in the corresponding section of this volume.

Without going into a detailed description of lichen symbiosis here, one should, however, note its originality. In this symbiosis, a biological unity of two organisms arises, which leads to the emergence of a third one that is fundamentally different from them. At the same time, each partner retains the features of the group of organisms to which it belongs, and none of them has a tendency to transform into an integral part of the other.

So far, lichens represent the only strictly proven case of the emergence of one completely new organism from two. This fact served as an impetus for the search for synthetic forms in a wide variety of systematic groups of plants and animals. However, all efforts made in this direction have so far been unsuccessful. Nevertheless, the assumption about the possibility of the existence of synthetic forms of organisms turned out to be so tempting that a new trend appears in biology. In contrast to the usual, firmly established views of biologists on the origin of organisms as a transition from simple to complex through differentiation, a new idea is emerging about the emergence of a complex organism from simpler ones through synthesis. Some biologists began to view the plant cell not as a product of gradual differentiation of the protoplast, but as a symbiotic complex synthesized from several simple organisms. These ideas originated and were most fully developed in the works of our domestic scientists.

First suggestion of an important role formative symbiosis(i.e., symbiosis leading to the formation of new forms) in the evolution of organisms was expressed by Academician A. S. Famintsin in 1907. Developing these thoughts further, K. S. Merezhkovsky in 1909 formulated the hypothesis of the symbiogenic origin of organisms and called it "theory of symbiogenesis". Later it became widely known among biologists. In the 20s, it was supported and further developed by the famous Soviet botanist B. M. Kozo-Polyansky. Nowadays, these ideas, already at a new level of development of biology, were revived by the American researcher Sagan-Margulis in her hypothesis of the origin of eukaryotic cells. In accordance with this hypothesis, cellular organelles such as mitochondria, basal bodies of flagella and plastids of eukaryotic cells arose from symbiotic prokaryotic cells of blue-green algae and bacteria. As the main argument, some similarities in the composition, structure and behavior of the listed organelles and prokaryotes are given. Undoubtedly, these facts deserve the closest attention. However, they are insufficient to substantiate the hypothesis of symbiogenesis, since similarities, as is known, can appear in structures or organisms of different origins and due to parallelism in evolution. Thus, the stigma-flagellum system in golden, yellow-green and brown algae is very similar in appearance and function to the rods of the retina of animals, although the entire process of their formation and the course of ontogenetic development indicate that there can be no talk of a common origin of these formations.

The study of symbiotic organisms in an electron microscope shows that even in such a highly developed symbiosis as glaucocystis, the partners retain their individual characteristics and autonomy. Analysis of symbioses of algae with various organisms reveals a certain direction in the development of relations between partners, mainly along the lines of maximum specialization of functions and the structural rearrangements caused by this circumstance while maintaining them as independent organisms; this goes against the provisions of the symbiogenesis hypothesis. All this indicates that currently the symbiogenesis hypothesis is at that stage of development when logical constructions clearly prevail over facts.

Of course, symbiosis can lead to the creation of new organisms, as evidenced by the appearance of such a unique plant group as lichens. The role of symbiosis in evolution cannot be denied. And yet it is obvious that this is not the only and not the main way of the formation of new forms of life. On the one hand, the fact of the existence of lichens leads to this conclusion, since they form an extremely specialized and isolated group of organisms, representing a blind branch of phylogenetic development. On the other hand, a large amount of factual material is now accumulating on the fine organization of the cell. It makes it possible to reconstruct a picture of the probable separation and complexity of the organization of some cellular organelles in algae. By the way, it was the absence of this kind of facts that at one time stimulated the emergence of the symbiogenesis hypothesis.

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