Ecological Association/Interactions among Soil Microorganisms
Soil is the largest terrestrial ecosystem where a wide variety of relationships exists between different types of soil organisms. The associations existing between different soil microorganisms, whether of a symbiotic or antagonistic nature, influence the activities of microorganisms in the soil. Microflora composition of any habitat is governed by the biological equilibrium created by the associations and interactions of all individuals found in the community. In soil, many microorganisms live in close proximity and interact among them-selves in a different ways. Some of the interactions or associations are mutually beneficial, or mutually detrimental or neutral. The various types of possible interactions/associations occurring among the microorganisms in soil can be: a) beneficial i) mutualism ii) commensalisms and iii) proto-cooperation or b) detrimental / harmful – i) amensalism, ii) antagonism, iii) competition iv) Parasitism and v) predation
a) Beneficial Association/Interactions:
Mutualism (Symbiosis): It is a relationship or a type of symbiosis in which both the interacting organisms/partners are benefited from each other. The way/manner in which benefit is derived depends on the type of interactions. When the benefit is in the term of exchange of nutrients, then the relationship is termed as "syntrophism" (Greek meaning: Syn -mutual and trophe = nourishment), for example, Lichen (association of algae or BGA with fungus) in which algae benefits by protection afforded to it by the fungal hyphae from environmental stresses, while the fungus obtain and use CO2 released by the algae during photosynthesis. Where the blue green algae are the partners in the lichen association, the heterotrophs (Fungus), benefit from the fixed nitrogen by the blue green algae.
Microorganisms may also form mutualistic relationships with plants, for example nitrogen fixing bacteria i.e. Rhizobium growing in the roots of legumes. In this Rhizobium-legume association, Rhizobium bacteria are benefited by protection from the environmental stresses while in turn plant is benefited by getting readily available nitrate nitrogen released by the bacterial partner.
The Anabaena-Azolla is an association between the water fern Azolla and the cyanobacterium Amabaena. This association is of great importance in paddy fields, where nitrogen is frequently a limiting nutrient.
An actinorrhizal symbiosis of actinomycetes, Frankia with the roots of Alnus and Casurina (non-legumes) is common in temperate forest ecosystem for soil nitrogen economy. Another type of symbiotic association which exists between the roots of higher plants and fungus is Mycorrhiza. In this association fungus gets essential organic nutrients and protection form roots of the plants and allows them to multiply and in turn plants uptake phosphorus, nitrogen and other inorganic nutrients made available by the fungus.
2. Commensalisms: In this association one organism/partner in association is benefited by other partner without affecting it. For example, many fungi can degrade cellulose to glucose, which is utilized by many bacteria. Lignin which is major constituent of woody plants and is usually resistant to degradation by most of the microorganisms but in forest soils, lignin is readily degraded by a group of Basidiomycetous fungi and the degraded products are used by several other fungi and bacteria which can not utilize lignin directly. This type of association is also found in organic matter decomposition process.
3. Proto-cooperation: It is mutually beneficial association between two species / partners. Unlike symbiosis, proto-cooperation is not obligatory for their existence or performance of a particular activity. In this type of association one organism favor its associate by removing toxic substances from the habitat and simultaneously obtain carbon products made by the another associate/partner. Nutritional proto-cooperation between bacteria and fungi has been reported for various vitamins, amino and purines in terrestrial ecosystem and are very useful in agriculture. Proto-cooperative associations found beneficial in agriculture are : i) synergism between VAM fungus-legume plants and Rhizobium in which nitrogen fixation and phosphorus availability / uptake is much higher resulting in higher crop yields and improved soil fertility, ii) synergism between PSM-legume plants and Rhizobium and iii) synergism between plant roots and PGPR in rhizosphere where rhizobacteria restrict the growth of phytopathogens on plant roots and secrets growth promoting substances.
b) Detrimental (Harmful) Associations/Interactions:
1. Antagonism: It is the relationship in which one species of an organism is inhibited or adversely affected by another species in the same environment. In such antagonism, one organism may directly or indirectly inhibit the activities of the other. Antagonistic relations are most common in nature and are also important for the production of antibiotics. The phenomenon of antagonism may be categorized into three i.e. antibiosis, competition and exploitation.
In the process of antibiosis, the antibiotics or metabolites produced by one organism inhibits another organism. An antibiotic is a microbial inhibitor of biological origin. Innumerable examples of antibiosis are found in soil. For example, Bacillus Species from soil produces an antifungal agent which inhibits growth of several soil fungi. Several species of Streptomyces from soil produces antibacterial and antifungal antibiotics. Most of the commercial antibiotics such as streptomycin, chloramphenicol, Terramycin and cyclohexamide have been produced from the mass culture of Streptomyces. Thus, species of Streptomyces are the largest group of antibiotic producer’s in soil. Another example of antibiosis is inhibition of Verticillium by Trichoderma, inhibition of Rhizoctonia by a bacterium Bacillus subtilis, inhibition of soil fungus Aspergillus terreus by a bacterium Staphylococcus aureus.
2. Ammensalism: In this interaction /association one partner suppress the growth of other partner by producing toxins like antibiotics and harmful gases like ethylene, HCN, Nitrite etc.
3. Competition: As soil, is inhabited by many different species of microorganisms, there exists an active competition among them for available nutrients and space. The limiting substrate may result in favoring one species over another. Thus, competition can be defined as “the injurious effect of one organism on another because of the removal of some resource of the environment”. This phenomenon can result in major fluctuations in the composition of the microbial population in the soil.
For example, chlamydospores of Fusarium, Oospores of Aphanomyces and conidia of Verticillium dahlae require exogenous nutrients to germinate in soil. But other fungi and soil bacteria deplete these critical nutrients required for spore germination and thereby hinder the spore germination resulting into the decrease in population. Competition for free space has been, reported to suppress the fungal population by soil bacteria. Therefore, organisms with inherent ability to grow fast are better competitors.
4. Parasitism: It is an association, in which one organism lives in or on the body of another. The parasite is dependent upon the host and lives in intimate physical contact and forms metabolic association with the host. So this is a host -parasite relationship in which one (parasite) is benefited while other (host) is adversely affected, although not necessarily killed. Parasitism is widely spread in soil communities, for example, bacteriophages (viruses which attack bacteria) are strict intracellular parasites Chytrid fungi, which parasitize algae, as well as other fungi and plants; there are many strains of fungi which are parasitic on algae, plants, animals parasitized by different organisms, earthworms are parasitized by fungi, bacteria, viruses etc.
5. Predation: Predation is an association / exploitation in which predator organism directly feed on and kills the pray organism. It is one of the most dramatic inter relationship among microorganisms in nature, for example, the nematophagous fungi are the best examples of predatory soil fungi. Species of Arthrobotrytis and Dactylella are known as nematode trapping fungi. Other examples of microbial predators are the protozoa and slime mold fungi which feed on the bacteria and reduce their population. The bacteriophages may also be considered as predators of bacteria.