Symbiosis Between the Soil, Plant And Microbial Organisms
The soil as an ecosystem is a medium through which plants grow and also serves as a habitat for diverse species of living organisms. Interdependence occurs among these groups and their activities is influenced by the type of interactions existing between them. The composition of the microflora of each habitat is determined by the natural balance created by the associations and interactions of all living things in the community.
Symbiosis is a long-term interaction between two biologically different species. The term “Symbiosis” is commonly used as only applicable to mutualism relationships, but can also be considered ”as all interspecific relationships in the coexistence of different organisms" (Denton & Krebs). The ecological interaction between the soil, plant and living organisms covers both the beneficial and non-beneficial co-existence aspects. In a symbiotic relationship, the species that accommodate the other is called the host while the specie living on or inside the host is referred to as the symbiont which is always smaller than the host. According to the specific host example, each host can have a single symbiont, several, or even millions.
In the soil, living organisms live together and interact in different ways. Some interactions are mutually beneficial or mutually harmful while some are neutral. The different types of possible associations between microorganisms in soil may be:
- Beneficial: mutualism, commensalism and proto-cooperation
- Non-Beneficial: amensalism, antagonism, competition, parasitism and predation
This is a symbiotic relationship in which the two interacting partners benefit from each other simultaneously. The way in which the benefit is achieved depends on the nature of the interactions. An example is a relationship between Rhizobium bacteria and the root of a plant. The soil medium is full of bacteria that fix several nutrients useful for plants. Nitrogen is essential for the growth of plants; it is said to be the most crucial nutrient for general growth, which is necessary for relatively large amounts for the synthesis of proteins and amino acids. Part of it is recovered as organic nitrogen from the degradation of other organisms from the soil, part of which is provided by nitrogen-fixing bacteria in the soil.
There is a symbiotic relationship between nitrogen-fixing bacteria that form nodules which serve as habitat for bacteria to stick to. This is very common in leguminous crops such as beans, peas, peanuts, and clover. These symbiotic bacteria are known as Rhizobium. When rhizobia come into close contact with the roots of a plant to form nodules, the extended epidermal cells, called absorbent hairs, bend around the rhizobium. A passage is created within the cell, known as the thread of infection, and the cell wall is degraded, allowing the rhizobium to penetrate into the inner cells of the root and form a node. Rhizobia have also adapted to form a symbiosis with plants that form nodes by developing sufficient resistance to plant defense to grow in a formed nodule.
Another mutual symbiosis is the arbuscular mycorrhiza which occurs between the roots of higher plants and fungus. Fungi are heterotrophic organisms that absorb their food and also can easily absorb essential nutrients such as phosphorus and nitrogen. Plants are autotrophic and produce their food in the form of sugar and starch during photosynthesis. However, plants often have difficulty in absorbing many of the essential nutrient elements, especially nitrogen and phosphorus. Most plants allow and require mycorrhizal fungi to colonize their roots to maximize the ability of optimum plant growth. In this symbiotic relationship, the hyphae of the fungus significantly increase the surface area open to nutrients and water intake and maximize the access of the plants to these compounds and essential elements (Denison & Kiers). The plant in return supplies the fungus with carbohydrates needed for energy.
In this association, an organization/partner in the association benefits from another partner without influencing it. This usually happens in the soil regarding the degradation of complex molecules such as cellulose and lignin. One organism may act on a substrate that is not available to the second organism, but the decomposition results in the formation of by-product used by the second organism. An example is the fungi that are capable of degrading cellulose and producing glucose and organic acids. This can serve as a growth source for many bacteria and fungi that are not cellulolytic.
It is a mutually beneficial connection between the two species. In contrast to symbiosis, protocooperation is not compulsory for their survival. An example of protocooperation occurs between soil bacteria or fungi and plants that grow in the soil. None of the species depend on the relationship to survive, but all fungi, bacteria and higher plants participate in the formation of soil composition and fertility. Soil bacteria and fungi are interdependent and form nutrients essential for plant survival. The plants receive nutrients from the root nodules and the degradation of the organic matter. Plants benefit from the extraction of essential minerals and carbon dioxide. Beneficial proto-cooperation include
synergy between VAM fungus-legumes and Rhizobium, where nitrogen fixation and phosphorus availability/uptake are much higher, resulting in better plant growth. One way this synergy is evident between plant roots and PGPR(plant growth promoting rhizobacteria) in the rhizosphere is where rhizobacteria limit the growth of plant pathogens in plant roots and secrete growth-promoting substances that enhance the plant’s ability for growth.
The relationship in which one species of one organism is inhibited or negatively affected by another species in the same environment. In such an antagonism, one organism can directly or indirectly inhibit the other's activities. Antagonistic relationships are more common and are also important for the production of antibiotics.
In an antagonism, antibiotics or metabolites produced by one organism inhibit another organism. There are countless examples of antibiotics in the soil. For example, ground Bacillus spp produce an antifungal that inhibits the growth of several soil fungi. Several species of Streptomyces produce 80% of antibacterial and antifungal antibiotics and are the largest group of antibiotic producers in the soil (de Lima Procópio et al.).
In this interaction/association, one partner suppresses the growth of another partner by producing toxins (antibiotics) and harmful gases such as ethylene, HCN, nitrite, etc.
Since the soil is inhabited by many types of microorganisms, there is active competition between them for available nutrients and space (survival of the fittest). The limiting substrate may favor one species over another. This phenomenon can cause significant variations in the composition of the microbial population in the soil. For example, the competition between soil bacteria and Fusarium chlamydospores occurs when the bacteria degrade these essential exogenous nutrients needed by fusarium for spore germination thus, leading to a decline in the population. It has been reported that competition for free space suppresses the fungal community caused by soil bacteria as a result of biologically induced nitrogen deficiency (Marshall & Alexander 143).
This an association in which an organism lives in or on the body of another. The parasite is host-dependent and lives in close physical contact and forms a metabolic association with the host. Thus, it is a host-parasite relationship in which one (parasite) gains while another (host) is affected but might not die in the process. For example, bacteriophages viruses that attack bacteria are severe intracellular parasites of chytrid fungi that use algae, fungi, and other plants as hosts. There are many strains of fungi, parasitic algae, plants, animals that are parasitized by various organisms, worms are parasitized by fungi, bacteria, viruses, etc.
An exploit association in which the predatory organism feeds directly and kills the preying organism. For example, nematophagous fungi are the best examples of predatory soil-forming fungi. Arthrobotrytis and Dactylella species are known as fungi that destroy nematodes. Protozoa and slime mold fungi that feed on bacteria and reduce their populations are other examples of microbial predators. Bacteriophages can also be considered as bacterial predators
“Symbiosis.” Wikipedia, Wikimedia Foundation, 6 Sept. 2018, en.wikipedia.org/wiki/Symbiosis.
De Lima Procópio et al. "Antibiotics Produced By Streptomyces". The Brazilian Journal Of Infectious Diseases, vol 16, no. 5, 2012, pp. 466-471. Elsevier BV, doi:10.1016/j.bjid.2012.08.014.
Denison, R. Ford, and E. Toby Kiers. “Life Histories of Symbiotic Rhizobia and Mycorrhizal Fungi.” Current Biology, vol. 21, no. 18, 2011, doi:10.1016/j.cub.2011.06.018.
Denton, Kaleda, and Dennis L. Krebs. “Symbiosis and Mutualism.” Encyclopedia of Evolutionary Psychological Science, 2016, pp. 1–4., doi:10.1007/978-3-319-16999-6_3050-1.
Marshall, K. C., and M. Alexander. “Competition between Soil Bacteria and Fusarium.” Plant and Soil, vol. 12, no. 2, 1960, pp. 143–153., doi:10.1007/bf01377367.