The microorganisms that inhabit soil exhibit many different types of association or interaction.
Types of Microbial Interactions
There are three types of Microbial Interactions, found in soil.
1. Positive interaction :
- Mutualism
- Commensalism.
2. Negative interaction :
- Ammensalism (antagonism),
- Competition.
- Parasitism,
- Predation,
3. Neutral association :
1]. Positive Associations :
(a) Mutualism :
- This is a symbiotic association where both the partners are benefited.
- The manner in which the benefit is derived varies.
* Synergism :
- It is the mutualistic association where both the partners derive benefit from the association.
- The association is not obligatory.
- Both populations are capable of surviving Independently, although each gains advantage from the relationship.
* Syntrophism :
- It is a type of synergism where two species supply each other's nutritional needs, such as vitamins, amino acids, etc. For example,
- Association between Enterococcus faecalis and Lactobacillus arabinosus.
- L. arabinosus requires phenylalanine for growth, which is produced by E. faecalis.
- E. faecalis requires folic acid which is produced by L. arabinosus.
- In minimal medium both populations can grow together, but neither can grow alone.
* Rhizosphere effect :
- The region where roots and soil make contact is called the rhizosphere.
- It is a synergistic interaction between microorganisms and plants.
- From the association both the partners derive nutritional advantage.
- Association between Thiobacillus ferroxidans and Beljerinckia lacticogenes in medium which lacks carbon and nitrogen sources.
- T. ferroxidans can fix CO₂ and B. lacticogenes can fix N₂ , thus both can grow in minimal medium devoid of C and N source.
- This association is useful in bioleaching of copper from its ore.
- Pure culture of T. ferroxidans can extract 30% of copper from ore.
- B. lacticogenes can extract 10% of copper from the ore. Both the organisms in association can leach upto 70% copper from its ore.
* Lichens :
- Lichens represent mutualistic association between heterotrophic fungi and photosynthetic algae or cyanobacteria. They are usually found on rock surfaces.
- Algae being photoautotrophic utilise light energy and atmospheric CO₂ to produce organic matter.
- In some lichens the cyanobacterial partner can also fix atmospheric N₂. Fungi are benefited by getting nutrients produced by algal partners.
- Fungi on the other hand provide protection and produce enzymes that solubilize rock minerals making essential nutrients available for both the partners.
* Mycorrhizae :
- This is an intimate association between plant roots and fungi, where the latter serves as additional roots to acquire nutrients.
- A mycorrhiza (fungus -root) is a type of endophytic, biotrophic, mutualistic symbiosis found in many natural ecosystems.
- Several types of mycorrhizal associations are differentiated on the basis of degree of interaction between plant roots and fungi.
- For example, association between basidiomycetes and roots of forest trees.
- An (arbuscular mycorrhizal) association occurs in plants belonging to the families: Amaranthaceae, Pinaceae, Betulaceae, Cruciferaceae, etc.
Ectomycorhizae :
- The fungi penetrates the outermost layers of tree roots and grows on the outer surface of the root. The fungal mycelium forms a sheath around the root of plants.
- In this association fungi obtain nutrients from plants, and in return it gets water and minerals from the soil through fungi.
- Most of these fungi cannot be cultivated in absence of plants.
- The plant growth is adversely affected in absence of fungi.
Endomycorrhizae :
- In this association fungi grow within the cells of plant roots.
- Sometimes the fungi form branch like structure or specialized inclusions called vesicles and arbuscules inside the plant cells and so, are called vesicular arbuscular mycorrhizae (VAM).
- VAM fungi play an important role in increasing plant growth by increasing supply of phosphorous to host plant. Also make the plant more resistant to plant diseases.
- These arbusculars are digested by the plant cells and the nutrients released from the fungi are used by the plants.
- The fungi in turn obtain nutrients from the plant tissues.
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| Schematic diagram of Arbuscular Mycorrhizal Fungi with plant root and its hyphal extension in soil |
(b) Commensalism
- It is an association between organism in which one partner benefits, while the other partner is not affected.
- This occurs in soil with respect to degradation of cellulose and lignin. e.g., association occurs between fungi and bacteria in soil.
- The cellulose degrading fungi degrades cellulose, produce glucose and organic acids which is used by bacteria for their growth.
- Thus bacteria are benefited by the association and fungi are not affected.
- Commensalism also exists when a mixed culture of organisms cause degradation of complex molecules which cannot be done by Individual organism.
- E.g., pure cultures of microorganisms cannot degrade lignin in laboratory, but the mixed microbial flora can easily degrade lignin forest soil.
- Many commensal relationships are based on the production of growth factors.
- Many nutritionally fastidious bacteria in soil often depend on growth factors such as vitamins and amino acids released from other organisms.
2]. Negative associations :
(a) Amensalism / Antagonism :
- This is an association where one partner inhibits other organism and thereby gains advantage from the association.
- The process is called antagonism. The organisms are called antagonist.
- Their presence in soil is very important, because they produce certain inhibitory substances or antibiotics.
- These products affect the growth or survival of other organisms e.g.,
- Antagonism between bacteria and fungi. Staphylococus aureus produces a diffusable antifungal material that causes distortion and hyphal swellings of Aspergillus terreus.
- Pseudomonas aeruginosa produces pigment which Inhibits germination of Aspergillus spores.
- Filamentous actimomycetes are antagonistic for many bacteria. They produce antibiotics which inhibit the growth or competitive population in the soil.
- Certain fungi present in the soil produce cyanide causing toxic effect on other microorganisms.
- Some algae produce fatty acids with antibacterial effect.
- Other antagonistic metabolic products include methane, sulfides and other volatile sulfur compounds.
- Streptomyces and Myxobacteria produce potent lytic enzymes which destroy other cells by digesting their cell wall or other protective surface layers.
(b) Competition :
- The organisms, which require same nutrients and similar environmental conditions influence each other.
- They compete for nutrients, light, space, oxygen, etc.
- The organisms, which are adapted to the situation will survive and the rest will be inhibited or destroyed.
- Competitive interactions result in ecological separation of closely related organisms.
- Competition may also limit the growth of all organisms as compared to their potential growth.
(c) Parasitism :
- This is a relationship in which one organism lives Inside or on the surface of other organism at the expense of the other organism.
- One partner is called parasite and the other is called host.
- The parasite feeds on the cells, tissues or body fluids of host, hence it is always harmed.
- All plants, animals and microorganisms can be attacked by microbial parasites.
- e.g., Parasitic association between bacteria. Bacterium Bdellovibrio bacteriovorous present in soil and sewage is a parasite of gram-negative bacteria.
(d) Predation :
- An association in which one species of organism kills and eats another species.
- The predator ingests the organism which is a prey. Such predator-prey interactions are of very small or short duration.
- e.g., soil fungal are predators of nematodes; amoeba is a predator of bacteria.
- All these negative associations normally control population densities in soil.
- The soil fungus Artrobotrys conoides produces hyphae that form rings to trap protozoa and nematodes and digest it.
- Some fungi such as Trichoderma and Lactisaria species can destroy other plant pathogenic soil fungi. Such mycoparasitic fungi are used as biopesticides to control plant diseases.
3]. Neutral Associations :
- It is the association in which both partner do not exhibit positive or detrimental effect on each other.
- This type of association occurs when each partner can utilise different nutrients without producing end products which is inhibitory to other.
- The two partners do not compete for nutrition even if they are present in low concentrations.
- Such a condition may be transitory as conditions or the relationship might change with variation in environmental conditions.
