We shall now briefly consider some specific symbiotic associations.
Photosynthetic and Non-photosynthetic Partners
The photosynthetic partner is the provision of nutrients. The function of non-photosynthetic one varies, which may be of nutrients (nitrogen-fixing root-nodule bacteria), of protection (fungal partner of a lichen), or of favorable position (tridacna clams housing algae in their mantles). Such associations may be categorized as follows:
1. Photosynthetic partner is a higher plant. The following types of associations belong to this category:
(a) Rhizosphere and phyllosphere. The rhizosphere is the region of the soil immediately surrounding the roots of a plant. Some authors include the root surface-rhizoplane also under the rhizosphere. From an operational viewpoint, the rhizosphere can be defined as the region extending a few millimeters from the root surface in which the microbial population of soil is influenced by the chemical activities of plants. The major effect is quantitative: the number of microorganisms in the rhizosphere usually is more than in the neighboring soil.
There is also a qualitative effect: short Gram-negative rods predominate in the rhizosphere, while Gram-positive rods and coccoid forms are less numerous in the rhizosphere than elsewhere in the soil. Root exudates in the rhizosphere are rich in soluble sugars, amino acids, vitamins, and other factors that serve as nutrients for microbes in the rhizosphere. (Growth & Distribution of Microorganisms Notes)
Similar associations exist in the phyllosphere or more precisely, the phylloplane, the surfaces of living leaves. The surface harbors a variety of microflora. The leaf leachates are the nutrients for microbes. There are both, quantitative and qualitative effects in the phylloplane.
(b) Mycorrhizae. The roots of higher plants are infected by fungi. The composite root-fungus structure is called mycorrhiza. There is an invasion of roots by a soil fungus. The hyphae penetrate the root cells by haustoria and develop intracellularly. In VAM (vesicular-arbuscular mycorrhiza) the fungus forms intracellular branching structures called arbuscules. A single species of pine may be associated with any of the 40 different fungi.
The fungal component is unable to use complex polysaccharides. The auxins excreted by the fungi induce a dramatic flow of carbohydrates from the leaves to the roots. The plants are said to be helped through the facilitation of the absorption of water and minerals from the soil. Thus pines absorb 2 to 3 times more phosphorous, nitrogen and potassium when mycorrhizae are present than when they are absent.
(c) Root nodule bacteria and legumes. It is known for a long time that the fertility of agricultural land can be maintained by crop rotation. Cultivation of leguminous plants could restore the declining levels of land productivity due to the cultivation of a single crop (say a cereal) for several years.
Root nodules contain a large number of bacteroids: small, rod-shaped or branched bodies (T, V, or Y-shaped) similar in size and shape to bacteria. M. W. Beijerinck isolated and cultivated the bacteria in 1888. The bacteria are Gram-negative motile rods that are classified in the genus Rhizobium.
Their numbers are very variable, depending upon the nature of the soil and its previous agricultural treatment. The soil under nonleguminous crops, such as wheat may have fewer than 10 Rhizobium cells per gram, the same soil will contain between 10 and 107 cells of Rhizobium following the cultivation of a legume crop. The ability of legume plants to stimulate the growth of Rhizobium in the soil extends as far as 10 to 20 mm from the roots. The effect is highly specific. The nature of the substance responsible for the stimulation is not yet very well known.
The infection of the root by Rhizobium resembles that by pathogens. Nevertheless, this association is not usually considered parasitism. The plant gains much more from the association. This is thus taken as mutualism.
(d) Root nodule bacteria and non-legumes. There are about 10 genera of non-legumes that form root nodules. These are as follows:
Coriaria (Coriariaceae), Myrica (Myricaceae), Alnus (Betulaceae), Casuarina (Casuarinaceae). Hippophae, Shepherdia and Eleagnus (Elaeagnaceae), Ceanothus and Discaria (Rhamnaceae), and Trema (Ulmaceae). The microbes in nodules are interpreted as actinomycetes on morphological grounds. The microbe in the nodule of Trema aspera is shown to be Rhizobium species.
2. Photosynthetic partner is a microorganism. The following types belong to this group:
(a) Zoochlorellae and zooxanthellae. These are the endosymbionts of protozoa. The green algae are present in freshwater forms of Ciliophora and Rhizopoda, whereas in marine forms there are generally yellow or brown algae. The two types of algae are called zoochlorellae and zooxanthellae respectively. Each protozoan cell harbors from 50 to several hundred algae. The algae resist digestion by the host. Alga converts CO2 to organic products through photosynthesis, liberating O2. The animal uses O2 for respiration and produces CO2 as a by-product. (Growth & Distribution of Microorganisms Notes)
(b) Cyanellae. These are blue-green bacterial endosymbionts of protozoa. They are found in a few genera of freshwater protozoa such as Cyanophoru, Peliaina, and Paulinelia. Paulinella contains from one to six cyancllae. The cyancllae was discovered by A. Pascher in 1929.
(c) Lichens. These are examples of the symbiosis of algae with fungi. A lichen is a composite organism, consisting of a specific fungus, usually an ascomycete, living in association with one or sometimes two species of algae or blue-green bacteria. The symbionts form a vegetative body or thallus. There are three morphological types of lichen thalli: crustose, adhering closely to the substrates (rock, tree bark), foliose, leaflike and more loosely attached to the substrate, and fruticose, forming pendulous strands or upright stalks.
The bulk of thallus is made up of fungal hyphae. The partners may be easily separated and grown in pure cultures. The algal partner may be modified by lichenization. Thus, certain filamentous blue-green bacteria do not form normal filaments in the thallus, but each cell is separately and surrounded by fungal hyphae. In culture, it forms filaments.
Algal symbionts belong to 26 genera: 17 green algae; 1 yellow-green algae and 8 blue-green algae. One green algal genus, Trebouxia is found in more than half of the described lichens. Most fungi are ascomycetes, and a few are Deuteromycetes and basidiomycetes. The lichen may remain viable in a dry state for months. They scavenge minerals by producing lichen acids, that dissolve as well as chelate the minerals. Chelation, the process of binding metal atoms to organic ligands, plays an important role in the solubilization and uptake of minerals by lichens. (Growth & Distribution of Microorganisms Notes)
Both partners are able to free-living existence. The association is, therefore, of mutual benefit only in very special ecological situations i.e. in environments where nutrients are extremely scarce and where extremes of wetting and drying occur. Under such conditions, the fungus benefits from alga for its source of organic nutrients. The contribution of the fungus is less clear. It may facilitate the uptake of water and minerals and may also protect the alga from desiccation and excess light intensities. (Growth & Distribution of Microorganisms Notes)
(d) Endosymbiosis of algae with aquatic invertebrates. Endosymbiotic algae have been recorded in more than 100 genera of aquatic invertebrates, particularly in the coelenterates (jellyfish, corals, sea anemones, hydra), the platyhelminths (flatworms, principally the planarians), the Porifera (sponges) and the molluscs (clams, squid). They are often found in the cytoplasm of cells concerned with digestion, as in the amoebocytes of sponges or the phagocytic blood cells of some clams. (Growth & Distribution of Microorganisms Notes)
Non-photosynthetic Partners
There are many symbioses where neither partner is photosynthetic. Most are cases of mutualism, and some are involved in parasitism also, as in the case of Bdellovibrio, a parasite of bacteria. This group has the following types:
1. Both are microorganisms. The following types are grouped under this category.
(a) Parasitism. The bdellovibrios, of which Bdellovibrio bacteriovorus is the type species are very small, Gram-negative bacteria bearing a single polar flagellum. They parasitize and kill other Gram-negative bacteria multiplying inside the host’s cell wall.
The life cycle is unique. It begins in a violent collision with a host cell. The cells move at high speed (100 cell lengths per second). Thus a host cell many times its size is carried a considerable distance by the momentum of impact.
They are a unique group of Gram-negative bacteria, very small in size (0.3 to 0.45 µm in diam.), with a sheathed flagellum, obligate aerobes, and obligate parasites of other Gram-negative bacteria. They have been found in soil samples from many parts of the world, as well as in sewage. Host-dependent species can be cultivated in a host-free medium containing an extract of host cells.
(b) Bacterial endosymbionts of protozoa. Bacteria have been described inside amoebae, flagellates, ciliates, and Sporozoa. None of them could be cultivated outside their host. Bacterial contributions to the host are unknown. In one case, it is said to provide amino acids and other growth factors to the host. The most extensively studied symbiosis is that between Paramecium, aurelia, and its endosymbiont, Kappa. The Kappa is the genetic element in the cytoplasm of Paramecium aurelia.
It is a bacterium-like an endosymbiont which divides by binary fission in the cytoplasm of paramecium and is responsible for the liberation of toxic particles in killer strains of paramecium. There are two strains of paramecium. There are two strains of paramecium killers which are immune to toxic substances and sensitives, to which these substances are lethal.
Kappa contains DNA and can undergo mutations. In addition to these, a number of other bacterial endosymbionts have been found in killer stocks of P. aurelia isolated from nature. These have also been designated by Greek letters. One of them, called alpha has affinities with Cytophaga. The endosymbionts perhaps synthesize the folic acid required by paramecium.
2. Microorganisms and the metazoan hosts. The following associations belong to this group:
(a) Ectosymbioses of protozoa with insects: the intestinal flagellates of wood-eating termites and cockroaches. The cellulose and lignin components of woody tissues of trees cannot be utilized by most animals, since they lack the enzymes for the degradation of these polymers. Nevertheless, many insects obtain their food from wood through an ectosymbiotic association with cellulose and lignin-digesting microorganisms. All wood-eating termites and cockroaches harbor flagellated protozoa (Polymastigotes and Hypermastigotes) in their gut. The flagellates digest cellulose which insects cannot.
The flagellates, in turn, are hosts to extracellular spirochetes and to intracellular bacteria, and perhaps some, if not all, of the cellulases may come from intracellular symbionts. Nitrogen fixation is also reported in the gut of termites. N2 – fixing bacteria may occur free in the gut or as intracellular symbionts of the flagellates.
The encystment of flagellates is regulated by the hormones of the cockroach. The hatching of eggs coincides with the peak of the molting season, and cyst formation in protozoan is induced by the molting hormone, ecdysone.
(b) Endosymbioses of fungi and bacteria with insects. Symbionts are never present in insects that have a nutritionally complete diet but are present in all insects that have a nutritionally deficient diet during their developmental stages. Thus no carnivorous insect has symbionts, whereas those living on blood or plant sap all contain symbionts. The main function of the symbiont is to provide one or more growth factors to the insect host that is absent from its diet. There are some exceptions, however, as in mosquitoes.
The endosymbionts are both bacteria and yeasts. In Rhodnius, the symbiont is an actinomycete of the genus Nocardia. Other symbionts are Gram-negative rods or coryneform. The specialized cells which contain symbionts are known as mycetocytes (for fungi) or bacteriocytes (for bacteria).
In cockroaches, symbionts provide the host with some essential amino acids. In some insects, they appear to help the host in the breakdown of nitrogenous waste products (uric acid, urea, xanthine, etc.)
(c) Ruminant symbiosis. Ruminants are a group of herbivorous mammals like cattle, sheep, goats, camels, giraffes, etc. The ruminant gut provides a more obvious example of mutualism. They use plant cellulose as a major carbohydrate source in their diet.
However, their normal gut cannot digest cellulose. Their digestive tract contains no less than four successive stomachs. They have developed a special region for cellulose digestion. This region is the rumen which is essentially a vast incubation chamber teeming with bacteria and protozoa. In cows, this resembles a large fermentation vat, about 100 liters, into which masticated plant materials enter for further digestion by the large number of anaerobic bacteria and protozoa. (Growth & Distribution of Microorganisms Notes Study Material)
The mutualistic microbes hydrolyze cellulose and other complex plant polysaccharides to their component monosaccharides which are then fermented to simply fatty acids (acetic, propionic, butyric) and gases (methane, carbon dioxide). The fatty acids are absorbed through the wall of the rumen into the bloodstream for use as carbon and energy source. The gases are passed out of the rumen at frequent intervals.
The microbial population of the rumen grows rapidly. These microbial cells pass out of the rumen along with undigested plant material into the stomach. These cells are destroyed and digested in the stomach through proteases (the rumen produces no digestive enzymes), in the normal way to provide essential amino acids and vitamins, etc. required for the growth of animals.
The cellulose-digesting bacteria of the rumen are all strict anaerobes. The species include Bacteroides succinogenes, Ruminococcus flavofaciens, R. albus, and Botryovibrio fibrisolvens. The great bulk of the bacterial population, however, is non-cellulolytic. Many of the rumen bacteria including some of the cellulolytic species are able of digesting starch, proteins, and lipids.
Only lignin of the ingested plant matter escapes digestion. The products of digestion of polysaccharides, proteins, and lipids are fermented by the rumen bacteria. During these processes, hydrogen gas combines with CO2 to form methane by Methanobacterium ruminantium.
(d) Ectosymbioses of microbes with birds: the honeyguides. The honeyguides, are a group of birds belonging to the genus Indicator. They are found in India and Africa. They guide honey badgers, as well as humans, to the nests of wild bees, where they wait for their followers to break open the hive. The honey guide feeds on the remnants of the exposed honeycomb after the badger or human has left.
The birds do not have enzymes for digesting beeswax. They harbor in their intestines two microbes, a bacterium, Micrococcus cerolyticus, and yeast, Candida albicans, which carry out digestion.
(e) Normal flora of the human body. There are many microbes in ectosymbiotic relationships with mammals. Both skin and mucous membranes are exposed to the external environment. These become contaminated at different stages and establish a normal flora. They are harmless unless under certain conditions. The throat and mouth support a variety of microbes including most of the eubacterial groups.
Gram-positive cocci (micrococci, pneumococci, Strepto-coccus salivarius) are common throat inhabitants. There are also Gram-negative rods of the genera Bacteroides and Spirillum. Gram-positive rods include mainly lactobacilli and corynebacteria. Spirochetes (Treponema dentium), yeasts (Candido Albicans), and actinomycetes are also common in the mouth. On the skin, there are present main corynebacteria, micrococci, non-hemolytic streptococci, and mycobacteria.
The stomach and small intestine are unsuitable environments for microbial development. The large intestine harbors a resident flora. In adult intestines, Bacteroides spp., E. coli, and Streptococcus faecalis are common. The yeasts are Candida and Toruiopsis. Protozoans are represented by Balantidium. Entamoeba (only in man) and Trichomonas.
Changing Nature of Symbiosis
It should be clear from the foregoing account that a variety of microorganisms are associated in different ways with other microbes, animals as well as plants. What are the effects of these associations? One type of effect may change with changing conditions. Thus a microbe associated with its host may not affect it in any way i.e. neutralism. However, under certain conditions the same microbe may affect its host adversely to produce some harmful effects and in extreme cases may also become parasitic i.e. parasitism.
Let us consider some systems in order to illustrate briefly the gradation from an apparently neutral relationship to a mutualistic or parasitic one. Let us consider three systems here-the human mouth, the mammalian gut, and the plant root.
1. Human mouth. We have seen above that the oral cavity has its own complex and characteristic microflora. The normal flora has high species diversity, being composed chiefly of Gram-positive cocci, rods, and filaments, but aerobic and anaerobic Gram-negative cocci and rods may also be found.
These microbes live in neutralism in the mouth without causing any harm. However, under the right conditions, they may become pathogenic and cause dental diseases. One such disease is caries, the localized dissolution of tooth enamel by acid.
The major organism involved in caries is Streptococcus mutans which utilizes dietary sucrose in two ways: to make extracellular poly-saccharides that aid in its adherence to tooth enamel; and in fermentation, to produce lactic acid. Thus there are set up localized sites of attack on the enamel. Caries, unlike other bacterial diseases, are the result of complex interactions between many different organisms living with and interacting with Streptococcus.
2. Mammalian gut. We have just seen above that the majority of microorganisms organisms in the gut occur in small and large intestines. The acids in the stomach prevent the growth and cause the death of microbes. Some tolerant bacteria can colonize the stomach lining, but their role is unknown. The concentration of microbes increases from the small to the large intestines and to the bowels. The gut in fact is like a continuous culture system with an entry part in the mouth and an exit port at the anus.
Most of these microorganisms could be now grown in the laboratory. Most of these were thought to be facultative anaerobes like E. coli. However, it appears that most gut microbes are very strict anaerobes, being killed by even the small amounts of oxygen used in the laboratory subculture. Special methods are used for their isolation. (Growth & Distribution of Microorganisms Notes Study Material)
What is the effect of these microbes on the host? Once, they were thought to be neutral. But it has now been shown that they are mutualistic symbionts. The germ-free animals have poorly developed defense mechanisms, have an abnormal gut, and suffer increased nutritional needs compared with normal animals. Some of these effects can also be produced in humans who are administered wide-spectrum antibiotics. The majority of flora is killed. We have already described the more obvious example of mutualism in the gut of ruminants.
3. Plant root. Generally, healthy leaves and stems do not provide a suitable niche for a large amounts of microbial growth but they do have their own characteristic microflora. The roots, however, are surrounded by the region, rhizosphere where nutrients for microbes are available in excretion products from the root. Environmental conditions in the rhizosphere do not fluctuate much as in the phyllosphere (for leaf).
The number of microbes is greater in the rhizosphere than in the soil away from the influence of roots. What do rhizosphere microbes do for plants? Their action was thought of as neutral. However, they are shown to play important role in antagonizing the growth of potential root pathogens. (Growth & Distribution of Microorganisms Notes Study Material)
A more organized relationship is with the fungi, resulting in mycorrhizae. These are said to confer an advantage to plants growing in poor soils where they aid in the absorption of nutrients, especially phosphates, by increasing the area of absorption.
Two contrasting relationships between bacteria and plant roots exemplify the spectrum of interactions that we find in nature. Rhizobium and Agro-bacterium are closely related Gram-negative bacteria (they show very close DNA homology). Both infect roots to cause abnormal localized growths. The relationship between Rhizobium and leguminous plants is a good example of a well-developed mutualism.
Each root nodule may contain as many as 10° bacteria. A clover crop may fix as much as 350 kg of nitrogen per hectare in a season compared with only about 5 kg per hectare for free living nitrogen-fixer in temperate areas of the world or up to 60 kg per hectare by cyanobacteria in the tropics.
In contrast, Agro-bacterium gives an example of parasitism. It forms crown gall tumors. The bacteria enter through a wound and move between its cells which are stimulated to rapid division. The bacteria have a large plasmid, Ti (tumor-including) plasmid, part of which is transferred to the plant cell, and incorporated and expressed in the plant nuclear DNA. Thus, it is possible that this plasmid could be used to transfer genes to plant cells to produce genetically engineered plants. (Growth & Distribution of Microorganisms Notes Study Material)
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