The following points highlight the seven main types of symbiotic relationship that exists between organisms. The types are: 1. Mutualism 2. Parasitism 3. Amensalism 4. Competition 5. Predation 6. Proto-Cooperation 7. Commensalism.
Mutualism describes a relationship in which both associated partners derive some benefit, often a vital one, from their living together.
Table 33.1 is an attempt to summarise the main kinds of mutualistic associations; some of which are trivial and of scientific interest only but others such as Rhizobium-legume association, mycorrhizae, coral-microbial association, herbivore-microbial association and lichens are very important, or indispensible, both to the local ecosystem and on a world scale.
The most important and the best studied mutualistic association between microorganisms and plants is undoubtedly that between Rhizobium spp. and various legumes.
Mycorrhizae (Sing. Mycorrhiza):
Mycorrhizae represent a mutualistic symbiosis between the root system of higher plants and fungal hyphae. Frank, who first noted the existence of such a characteristic association in the roots of Cupulifereae in 1885, coined the term ‘mycorrhiza’. Over the last 20 years, basic works conducted by hundreds of researchers from different countries has shown that this association is fundamental and universally occurring.
Among the different symbiotic associations between the soil microorganisms and root of plants, mycorrhizae are the most prevalent as they occur on more than 90% of the vascular plants.
However, Kumar and Mahadevan (1984) have studied a large number of mycorrhizal associations and found that they are highly influenced by the toxic substances that, when present, are essentially concentrated in the root of plants. Such substances may be alkaloids, phenolics, terpenoids, tannis, stilbenes, etc.
The mycorrhizae are advantageous because:
(i) The fungus derives nutrients via the root of the plant. Sugars formed in the leaves move down the stem as sucrose. Sucrose itself never accumulates in the fungus; it is converted into isomers such as ‘trehalose’ thus resulting in the low sugar concentration,
(ii) The fungal hyphae act like a massive root hair system, scavenging minerals from the soil and supplying them to the plant, and
(iii) Due to this associationship the plant partner, in addition to the nutritional benefits, develops drought resistance, tolerance to pH and temperature extremes, and greater resistance to pathogens due to ‘phytoalexins’ released by the fungus.
Mycorrhizae are generally classified into two types, although a third type that is more or less a combination of the first two is recognized by some. The two major types are termed Ectomycorrhizae and Endomycorrhizae while the third one, however, is referred to as Ectendomycorrhizae.
Ectomycorrhizae (Ectotrophic mycorrhizae):
Ectomycorrhizae (Fig. 33.2) are common on many forest trees, particularly pines, beech and birch which are of much economic value. The fungal hyphae form a sheath over the outside of the roots which is generally called ‘mantle of hyphae’. From this mantle, a hyphal network called hartignet extends into the first few layers of the cortex or rarely deeper and then reaches the endodermis.
Root hair formation is suppressed in the infected root and the root morphology is changed by the repeated formation of short branches with blunt tips and limited growth. Common ectomycorrhizal genera are Basidiomycetes, particularly Agaricales such as Amanita, Tricholoma, Russula, Lactarious, Suillus, Leccinum and Cortinarius’, some Ascomycetes such as the truffles have also been reported.
The fungi of ectomycorrhizae secrete various growth promoting substances such as auxins, cytokinins and gibberellic acids. Nevertheless, they produce some antimicrobial substances which protect the host plant against soil-borne pathogens.
Fungi derive their carbon from the host in the form of glucose, fructose or sucrose which is ultimately converted to manitol, trehalose, and glycogen. These mycorrhizae are known to stimulate plant growth and nutrient uptake in soils of low to moderate fertility.
Ectomycorrhiza (Ectotrophic mycorrhiza):
The mycorrhizae in which the fungal hyphae invade the root cells without forming any external sheath, mantle of hyphae, are called endotrophic mycorrhizae. Usually, some part of invading fungal hyphae lie externally as a loose mass of hyphae but they do not form mantle.
Three types of endomycorrhizae are recognised:
(i) Vasicular arbuscular (VA),
(ii) Orchidaceous, and
(i) Vesicular-arbuscular (VA) mycorrhizae:
Vesicular-arbuscular (VA) mycorrhizae (Fig. 33.3) represent associations between fungi, mostly the members of Zygomycetes, and a great number of angiosperms such as tropical forest trees, almost all agricultural crops (except rice in paddy fields), and most of the herbs and grasses of tropical and temperate natural ecosystems.
Fungi forming VA mycorrhizae are restricted to only one family, Endogonaceae, of Zygomycetes with two genera, Endogone and Glomus, forming associations with a huge variety of distantly related plants. VA mycorrhizae are especially important because of their widespread occurrence and association with agricultural crops.
In VA mycorrhizae the fungal hyphae develop some special organs, called vesicles and arbuscules, within the root cortical cells. Vesicles are thick-walled, spherical to oval in shape, borne on the tip of the hyphae either in intercellular spaces or in the cortical cells of the root. These vesicles are food storage organs of the fungus.
However, the arbuscules are brush-like dichotomously branched (extensively) haustoria developed within the cortical cells. Though widely distributed geographically, the VA mycorrhizae are not of usual occurrence in continuously flooded sites (Keeley, 1980).
The importance of VA mycorrhizae is in the effects that they have on plant nutrition, especially the immobile elements such as phosphorus. The external hyphae greatly increase the volume of soil and translocate the phosphorus to the roots. Plants are heavily infected with VA mycorrhizal fungi in phosphorus-deficient soils and mycorrhizae are poorly developed when the phosphorus supply is adequate.
It is thus a self-regulating system, increasing phosphorus uptake when this element is in short supply. The phosphorus so absorbed is converted into polyphosphate granules in the hyphae and passed to the arbuscules for ultimate transfer to host plant. Gianinazzi et al. (1979) have demonstrated that transfer of polyphosphate occurs in the presence of acid phosphatase during the life span or scenescence of arbuscule.
In addition to stimulation of phosphorus uptake, mycorrhizal fungi stimulate rooting, growth, and survival of the transplant. Lambert et al. (1979) have studied that the VA mycorrhizae stimulate uptake of zinc, copper, sulfur, and potassium by the plant; enhances nodulation in legumes; decreases rots caused by fungal pathogen, and root penetration and larval development of nematodes.
(ii) Orchidaceous mycorrhizae:
Orchidaceous mycorrhizae (Fig. 33.4) are very different from VA mycorrhizae. Here the higher plant is temporarily or permanently parasitic on the fungus; the latter are mostly from the genus Rhizoctonia with the perfect stages occurring in basidiomycetes and ascomycetes. Orchid seeds are minute (0.3-14 μg), without any significant food reserve.
Some fail to germinate at all unless infected by fungus; others germinate, but, development soon ceases unless the seedling becomes infected by the fungus. The hyphae of the fungus penetrate the cells of the cortex and form coils within the cortical cells. These nutrient rich hyphal coils, generally called peletons, then break down making food available to the plant.
How the orchid persuades the fungus to undergo this bizarre self-sacrifice is not known, though the role of “phytoalexin orchinol” has been implicated. However, the degenerating peletons supply the orchid with carbohydrate and probably vitamins and hormones obtained by saprophytic action of fungus outside the root.
Most orchids eventually become green, and so the association between the orchid and the fungus may shift from parasitism to mutualism. Other orchids, e.g., Neottia nidus-avis (bird’s nest orchid) remain acholorophyllus and parasitic on the fungus throughout their life.
(iii) Ericaceous mycorrhizae:
Ericaceous mycorrhizae (Fig. 33.5) are associated with the two families, ericaceae and epacridaceae, in which the fungus forms dense intracellular coil in the outer cortical cells. Earlier, it was believed that the fungus was Phoma sp. but cultural studies proved that it is Pezizella ericae (an ascomycete).
This fungus has tremendous capacity of mineralization and absorption of organic nitrogen, thus it greatly stimulates nitrogen uptake and plant growth even in infertile peat soil. Englander and Hull (1980) have suggested Clavaria spp. as the mycorrhizal fungus of Rhidodendron spp. and Azalea spp. However, this mycorrhizal association result in no development of root hairs as well as the absence of epidermal cells of the root.
Ectendomycorrhizae (Ectendotrophic mycorrhizae):
The mycorrhizae which bear the characteristics of both, ecto- and endomycorrhizae, are categorised by some as ectendomycorrhizae. The fungal partner establishes mantle of hyphae on the surface of the root as well as hyphal coils and haustoria within the invaded cortical cells of the root.
In the forests, ‘Conifer-Boletus-Monotropa’ association represents a well studied example of ectendomycorrhizae. Monotropa, a nonchlorophyllous plant, usually grows near the roots of the conifers in the forests.
It is now well established that a fungus called Boletus forms a common mycorrhizal association between the conifer and Monotropa, though the nature of the association differs with the two plants. Boletus forms ectomycorrhiza with Monotropa and endomycorrhiza with the conifer. The fungus forms a bridge between the two plants.
Syntrophy (Gk. syn = together; trophe = nourishment) is such mutualistic interrelationship between two different microorganisms which together degrade some substances (and conserve energy doing it) that neither could degrade separately.
In most cases of syntrophism the nature of a syntrophic reaction involves H2 gas being produced by one partner and being consumed by the other. Thus, syntrophy has also been called interspecies hydrogen transfer.
Following are some examples of syntrophic associations:
(i) Ethanol fermentation to acetate and eventual production of methane (Fig. 33.6.) is a good example. Ethanol oxidizing bacterium ferments ethanol producing H2 which is a valuable electron donor for methanogenesis hence used by a methanogen. When both these reactions are summed, the overall reaction is exergonic (i.e., energy releasing).
Actually, the oxidation of ethanol to acetate plus H2 is energetically unfavourable, the reaction becomes favourable when H2 produced during it is consumed by the methanogens. In this way, both partners thus use the energy released in the coupled reaction of syntrophic association.
(ii) Oxidation of butyric acid to acetic acid plus H2 by the fatty acid-oxidizing syntroph Syntrophomonas is another good example. Syntrophomonas does not grow in a pure culture on butyric acid as the energy released during butyric acid oxidation to acetic acid is highly unfavourable to the bacterium. But, if the hydrogen produced in the reaction is immediately utilized by a syntrophic partner (e.g., methanogen), Syntrophomonas grows luxuriantly in mixed-culture with the H2 consumer.
Butyrate– + 2H2O → 2 Acetate– + H+ + 2H2 + Energy (+ 48.2 kJ)
(iii) Syntrophobacter degrades low molecular weight fatty acids (e.g., propionic acid) to produce H2 which, if not consumed, inhibits the growth of the producer. When this hydrogen is immediately consumed by Methanospirillum, the syntrophic partner, the growth rates of both Syntrophobacter and Methanospirillum is stimulated, i.e., both partners are benefited.
Corals are highly productive and yet live in waters that are very poor in nutrients; the open ocean may have a net productivity of 50 g cm-2 year-1 whereas coral reefs may produce up to 2500 g cm-2 year-1. The reasons for this are still not clear.
It is considered that the dinoflagellate symbionts, Gymnodinium microadriaticum and Amphidinium species, are ubiquitous in reef-building corals and they pass atleast 25%, and probably as much as 60 to 70% of their fixed carbon to the animal as glycerol and glucose.
They may also take up nitrate from the water and pass it to the coral in a utilizable form alanine. Cyanobacteria are important on reefs in fixing nitrogen, and they may be free living or symbiotic.
Bacterial symbionts living on the outside of the coral in mucilage layer have also been implicated in the conservation and rapid recycling of phosphorus and nitrogen to the corals. There are, therefore, a number of potential or actual symbiotic microorganisms which could account for the productivity of coral reefs.
Apart from the productivity aspects, Gymnodinium is also very important in depositing skeletal calcium as a result of photosynthesis.
The dinoflagellates apparently have to reinfect each generation of corals for they are not passed on during reproduction. How they do this is not known for they have never been found free living in the environment, though they can grow independently in culture.
Plants contain about 30% cellulose (dry weight), the large insoluble inert polysaccharide. It would be very much to the advantage of any herbivore to digest the chemical but, however, the only herbivore to possess the appropriate digestive enzyme, cellulase, are snails.
All others, from insects to mammals, do not possess this enzyme and they establish mutualistic associationship with cellullose-splitting bacteria and protozoa. These microorganisms generally occupy one of the several sites in the gut, the most advanced condition being that in ruminants.
Ruminants, such as cow and sheep, have evolved a unique four chambered ‘stomach’ that has helped establish them as extremely successful herbivores (Fig. 33.7). The rumen volume is large compared with size of the mammal (in the cow it is 80-100 L) so that there being a long resistance time for cellulose decomposition. Plant material is chewed, mixed with saliva and passed to rumen.
The rumen contains very numerous microorganisms of which about 90% are cellulase secretors. These may be 104 to 105 protozoa, 1010-1011 bacteria and 4 x 104 fungi. The contents of the rumen are continually mixed by slow contractions of the wall at 1-2 minute intervals.
Microbial Action in the Rumen of Ruminants:
The cellulases hydrolyse the cellulose to glucose, and the microorganisms then ferment the glucose to a variety of organic acids such as acetic acid (ethanoate), butyric acid (butyrate) and propionic acid (propionate), so providing energy for their own growth (Fig. 33.8).
About 103 dm3 per day of CO2 and CH4 are produced as waste products, which are burped out by the animal. Thus the microorganisms present in rumen convert largely indigestible plant material into low molecular weight carbon compounds which can be utilized by the herbivore.
Lichens are remarkable in that under natural conditions the algal-fungal or cyanobacterial-fungal association behaves as a single organism. The fungus (mycobiont) is usually an ascomycete and about 20,000 lichen fungi have been described which is approximately 25% of all known fungi. There are only some 30 genera of algae (photobiont; earlier called phycobiont) and cyanobacteria (cyanobiont) known to form lichens.
The relationship between the two associates of the lichen thallus is still not fully confirmed, though lichens have been the classic material for the study of microbial mutualistic symbiosis. The phycobiont/cyanobiont supplies carbohydrate to the mycobiont and the latter may supply minerals to the former.
We have no experimental confirmation that the mycobiont supplies minerals to its associates; also, the phycobiont may be able to absorb its own minerals from the substrate. ‘Good’ laboratory conditions cause the association to break down, whilst adverse conditions help to maintain it. This indicates that the association probably enables the associates to exploit habitat which would be unsuitable when they grow apart.
Lichens are considered the ‘pioneer organisms’ as they have been claimed to be important in increasing the rate of soil formation from bare rock. They may accelerate physical destruction of the rock by shrinkage and expansion of the thallus, may decompose the rock by wide range of chemical substances such as carbon dioxide (acting as H2CO3), various organic acids, and chelating agents.
Lichens may accumulate minerals and nitrogen which are eventually released to the primitive soil when the lichen thallus is decayed. Lichens are greatly effected (even killed) by the level of SO2 present in the atmosphere; their abundance can be used as an indicator of atmospheric pollution. They or their products may be used as food dyes, and indicators (litmus).
Parasitism represents the symbiotic associationship between two living organisms and is of advantage to one of the associates (parasite) but is harmful to the other (host) to a greater or lesser extent. The parasites may be destructive or balanced. The former destroy the host cells in their later stages of development whereas the latter fulfill their demands from the host in such a way that the host cells are not destroyed but continue to live.
Facultative and Obligate Parasites:
Associations would be easy to describe if organisms always behaved in the same way. Unfortunately, they do not. Many microorganisms, for instance, can survive as both parasites and saprophytes.
The fungus Ceratocystis ulmi, which causes Dutch elm disease, kills the tree and then lives saprophytically on its dead remains. Such an organism which mostly lives as saprophyte but seldom holds the charge of a parasite is referred to as facultative parasite.
In contrast, downy mildews, powdery mildews, etc. only grow on live protoplasm of the host plant in nature. Such as organism which cannot live elsewhere except on the living protoplasm of its host in nature is called obligate parasite (biotroph). Facultative and obligate parasites often differ in their pathogenic effects, i.e., in their ability to injure the host.
Since obligates are restricted to living organisms, their effects on the host are often less severe, although the host may show less vigorous growth. In contrast, facultative parasites which have only recently acquired a host, tend to be more damaging.
When one fungus parasitizes the other, the act is referred to as ‘mycoparasitism’. This term has been generally used interchangeably with ‘hyperparasitism’, ‘direct parasitism’ or ‘interfungus parasitism’. The incitant is generally called ‘mycoparasite’ or ‘hyperparasite’.
Mycoparasitism has been classified into two main groups on the basis of nutritional relationship of parasite with host:
(i) Necrotrophic and
The necrotrophic (destructive) parasite makes contact with its host, excretes a toxic substance which kills the host cells and utilizes the nutrients that are released. The biotrophic (balanced) parasite is able to obtain its nutrients from the living host cells, a relationship that normally exists in nature.
The mycroparasitism is of common occurrence and examples can be found among all the groups of fungi from chytrids to higher basidiomycetes. Few examples are as follows.
A three member mycoparasitic associationship has also been reported in which Chytridium parasiticum is parasite on Chytridium subercrelatum which, too, parasitizes Rhizidium richmondense, another chytrid.
The biological control of plant diseases has recently become an area of intensive research in view of the hazardous impact of pesticides and other agro-chemicals on the ecosystem. Amongst the biological agents, the mycoparasites have attained a significant position.
It has been suggested that efforts should be made to investigate the biological control of plant diseases through parasitism and predation. Therefore, the mycologists and plant pathologists are searching for new mycoparasites because the greater number of these the greater would be the chance of exploiting them as agents for biological control. Trichoderma is an important example.
Amensalism (from the Latin for not at the same table) refers to such an interaction in which one microorganism releases a specific compound which has a negative effect on another microorganism. That is, the amensalism is a negative microbe-microbe interaction.
Some important examples are the following:
(i) Antibiotic production by a microorganism and inhibiting or killing of other microorganism susceptible to that antibiotic is the most important example of amensalism. Concentrations of such antibiotics in the bulk of soil or water are certainly small, though there could be a large enough quantity on a micro-habitat scale to give inhibition of nearby microorganisms.
The antibiotics reduce the saprophytic survival ability of pathogenic microorganisms in soil. The attini ant- fungal mutualistic relationship is promoted by antibiotic producing bacteria (e.g., Streptomyces) that are maintained in the fungal gardens (see box). In this case, Streptomyces produces an antibiotic which controls Escovopsis, a persistent parasitic fungus, which can destroy the ant’s fungal garden (Fig. 33.9).
(ii) Production of ammonia by some microbial population is deleterious to other microbial populations. Ammonia is produced during the decomposition of proteins and amino acids. A high concentration of ammonia is inhibitory to nitrite oxidizing populations of Nitrobacter.
In contrast to the positive interactions of mutualism and synergism, competition represents a negative relationship between two populations in which both populations are adversely affected with respect to their survival and growth. In this case, the microbial populations compete for a substance which is in short supply.
Competition results in the establishment of dominant microbial population and the exclusion of population of unsuccessful competitors. During decomposition of organic matter the increase in number and activity of microorganisms put heavy demand on limited supply of oxygen, nutrients, space, etc.
The microbes with weak saprophytic survival ability are unable to compete with other soil saprophytes for these requirements and either perish or become dormant by forming resistant structures.
Predation typically occurs when one microorganism, the predator, engulfs and digests another microorganisms, the prey, and the former derives nutrition from the latter. In microbial fraternity, however, the distinction between predation and parasitism is not sharp.
The interaction between Bdellovibrio bacteria and other small gram-negative bacteria is considered by some as predation but by others as parasitism. Bdellovibrio is apparently quite widespread in aquatic habitats and attacks other bacteria, normally gram-negative ones, by boring a hole in the wall, entering the bacterium and causing lysis with the eventual release of many small vibrio-shaped bacteria.
The major microbial predators are the protozoa which may engulf bacteria and more rarely algae and other protozoa. These systems have been used extensively in models and simulations of predator-prey-relationship. In the simplest form the protozoan population (e.g., Tetrahymena) is limited by its bacterial food (e.g., Klebsiella) and numbers of both prey and predator show cyclic oscillations.
Another such example is of Didinium-Paramecium (both protozoa) relationship. Didinium preys on the Paramecium until the population of the later becomes extinct. Lacking a food source, the Didinium population also becomes extinct.
If a few members of the Paramecium population are able to hide and escape predation by the Didinium, then the Paramecium population recovers following the extinction of the Didinium. Thus, a cyclic oscillation can occur in the population of these two protozoans.
Predatory fungi exist and have been considered as possible bio-control agents for some diseases of plants caused by soil microorganisms. Nematodes and protozoa may be trapped by a variety of net-like-hyphae, sticky surfaces and nooses. The organism is then invaded by hyphae and digested.
6. Proto-Cooperation (Synergism):
Proto-cooperation (or synergism), like mutualism, represents an association between two microbial populations in which both populations benefit from each other, but it differs from the mutualism in that the association is not ‘obligatory’.
Both synergistic populations of microbes are able to survive in their natural environment on their own. Proto-cooperation or synergism allows microbial populations to perform metabolic activities such as synthesis of a product which neither population could perform alone.
Following are few examples:
(i) The Desulfolvibrio bacteria supply H2S and CO2 to Chlorobium bacteria and, in turn, the Chlorobium bacteria make sulphate (SO4–) and organic material available to Desulfovibrio. Thus the mixture of the two bacterial populations produce much more cellular material than either alone (Fie 33.10A).
(ii) Nocardia populations metabolize cyclohexane resulting in degradation products that are used by Pseudomonas population. The Pseudomonas species produce biotin and growth factors that are required for the growth of Nocardia (Fig. 33.1 OB).
(iii) Azotobacter populations present in soil fix atmospheric nitrogen if they have a sufficient source of organic compounds. Other soil bacterial populations such as Cellulomonas are able to utilize the fixed form of nitrogen and provide the Azotobacter populations with needed organic compounds (Fig. 33.10C).
Commensalism represents a relationship between two microbial populations in which one is benefited and the other remains unaffected (i.e., neither benefited nor harmed). Thus the commensalism is an unidirectional relationship between two microbial populations. It is quite common, frequently based on physical or chemical modifications of the habitat, and is usually not ‘obligatory’ for the two populations involved.
Commensalistic association is often established when one microbial population, during the course of its normal growth and metabolism, modifies the habitat in such a way that the other population is benefited.
Following are some examples:
(i) A disease causing microbial population when opens a lesion on the host surface, if creates an entry- passage for other microbial population that otherwise could not enter and grow in the host tissues. For convenience, Mycobacterium leprae, the causative agent of leprosy, opens lesions on the body- surface and thus allows other pathogens to establish secondary infections.
(ii) When facultative anaerobes utilize oxygen and lower the oxygen content, they create anaerobic habitat which suits the growth of obligate anaerobes because the latter benefit from the metabolic activities of the facultative anaerobes in such a habitat.
On the contrary, the facultative anaerobes remain unaffected. The occurrence of obligate anaerobes within habitats of predominantly aerobic character, such as the oral cavity, is dependent on such commensal relationship.
(iii) Population of Mycobacterium vaccae, while growing on propane cometabolizes (gratuitously oxidizes) cyclohexane to cyclohexanone which is then used by other bacterial population, e.g., Pseudomonas (Fig. 33.11). The latter population is thus benefited since it is unable to oxidise cyclohexane to cyclohexanone. Mycobacterium remains unaffected since it does not assimilate the cyclohexanone.
(iv) Some microbial populations create commensalistic habitat by detoxifying compounds by immobilization. Leptothrix bacteria deposite manganese on their surface. In this way, they reduce manganese concentration in the habitat thus permitting the growth of other microbial populations. If Leptothrix do not act so, the manganese concentration would be toxic to other microbial populations.