Types of Microbial Interactions in Soil : Mutualism, Commensalism, Amensalism, Compitition, Parasitism, Predation & Neutral association

  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.

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.

1. 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.