Microbial Interactions Among Soil Microorganisms

Interactions among soil Microorganisms

The microbial ecosystem is the sum of biotic and abiotic components of soil. it includes the total microbial flora together with the physical composition and physical characteristics of the soil. The microorganisms that inhabit the soil exhibit many different types of interactions or associations. Some interactions are indifferent or neutral;while some are positive and some are negative in nature. The associations existing between different soil microorganisms, whether of a symbiotic or antagonistic nature, influence the activity of microorganisms inthesoil.

Neutral association

Neutral association or neutralism is the association between microorganisms, where two different species of microorganisms occupy the same environment without affecting each other. Such an association might be transitory; as condition change in the environment, like nutrients availability, there might be a change in the relationship.

Positive associations

There are three types of positive association exist between microorganisms, which are below:

Mutualism

It is the symbiotic relationship in wich each organism is benefited from the association. This is an obligatory relationship in which each organism are metabolically dependent on each other. Lichens are the association between specific ascomycetes (the fungus) and certain genera of either green algae or cyanobacteria. In a lichen, the fungal partner is termed the mycobiont and the algal or cyanobacterial partner, the phycobiont. Because the phycobiont is a photoautotroph- dependent only on light, carbon dioxide, and certain mineral nutrients- the fungus can get its organic carbon directly from alga or cyanobacterium. The fungus often obtains nutrients from its partner by haustoria ( projections of fungal hyphae) that penetrate the phycobiont cell wall. it also uses the O₂produced during phycobiont photophosphorylation in carrying out respiration. In turn, the fungus protects the phycobiont from excess light intensities, provides water and minerals to it, and creates a firm substratum within which the phycobiont can grow protected from environmental stress.

Phycobiont- Cyanobacteria ( Chlorophyta, xanthophyta)

Mycobiont- Ascomycota, Basidiomycota

TheRhizobiumin the root nodules, where they differentiate morphologically into bacteroids, fix nitrogen from the atmospheric N_2,into a plant-usable form, ammonium, using the enzyme nitrogenase. In return, the plant supplies the bacteria with carbohydrates, organic acids (principally as the dicarboxylic acids and succinate) as a carbon and energy source, proteins, and sufficient oxygen.

Proto-cooperation/ Synergism

Protocooperation is a mutually beneficial relationship, similar to that which occurs in mutualism, but in protocooperation, this relationship is not obligatory. An example of protocooperation happens between soil bacteria or fungi, and the plants that occur growing in the soil. None of the species rely on the relationship for survival, but all of the bacteria, fungi, and higher plants take part in shaping soil composition and fertility. Soil bacteria and fungi interrelate with each other, forming nutrients essential to theplantssurvival. Plant utilize these microorganism synthesized nutrients through root nodules thereby decomposing organic substances. Soil bacteria and fungi help in improving the fertility of the soil and shaping of the soil. Plants get essential carbon dioxide and nutrients. Nutritional protocooperation between bacteria and fungi has been reported for various vitamins, amino and purines in the terrestrial ecosystem are very useful in agriculture.

Commensalism

Commensalism is a relationship in which one symbiont,the commensal, benefits while other (sometimes called the host) is neither harmed nor helped. This happens commonly in soil with respect to degradation of complex molecules like cellulose and lignin. For example, many fungi can degrade cellulose to glucose, which is utilized by many bacteria. Many bacteria are unable to utilize cellulose, but they can utilize the fungal breakdown products of cellulose, e.g., glucose and organic acids.

Another example of commensalism is that of a change in the substrate produced by the combination of species and not by individual species. For example, lignin which is the major constituents of woody plants and is usually resistant to degradation by most of the microorganisms. But in forest soils, lignin is readily is degraded by a group of Basidiomycetous fungi and the degraded products are used by several other fungi and bacteria which cannot utilize lignin directly.

Negative associations

Antagonism

It is the relationship in which one species is inhibited or adversely affected by another species in the same environment. The relationship is known as antagonism. The species which adversely affects other is said to be antagonistic. such organisms may be of great practical importance since they often produce antibiotics or the other inhibitory substances which affect the normal growthprocessesorsurvival of other organisms.

Antagonistic relations are most common in nature. One example of which is the antagonistic nature of bothStaphylococcus aureusandPseudomonasaeruoginosatowardsthe fungusAspergillus terreus.

Competition

Soil is inhabited by different kinds of microorganisms, and therefore they exhibit competition among themselves for nutrients and space. In this kind of situation, the best adapted microorganism will predominate or eliminate the others which are dependent upon the same limited nutrient substance. The organisms with inherent ability to grow fast are better competitors.

Exogeneous nutrients are required for the germination of chlamydospores ofFusarium, Oospores ofAphanomycesand conidia ofVerticillium dahlaein 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. Soil bacteria compete for space and suppress the growth of the fungal population.

Parasitism

Parasitism is the relationship between two organisms, in which one organism lives in or on another organism. The parasite is dependent upon the host and feeds on the cells, tissues or fluids of the host organism. The parasite lives in intimate physical contact with the host and forms metabolic association with the host. All major groups of plants, animals, and microorganisms are susceptible to attack by microbial parasites.

The bacterial parasite of Gram-negative bacteriaBdellovibrio bacteriovoruswhich is widespread in soil and sewage attaches to a host cell at a special region and eventually causes the lysis of that cell. As a consequence, plaque like areas of lysis appear when these parasites are plated along with their host bacteria. Parasitism is widely spread in soil communities. Viruses which attack bacteria (bacteriophages), fungi, and algae are strict intracellular parasites since they cannot be cultivated as free-living forms. There are also many strains of fungi which are parasitic on algae and other fungi by penetration into the host. Fungi with antagonistic activity toward plant pathogens have an essential role in plant growth and health. Mycoparasites and presumptive mycoparasites have biocontrol potential, some are responsible for natural suppressiveness of soils to certain plant pathogens. Several species ofTrichodermawere used successfully against certain pathogenic fungi.Trichodermasp. was used as commercial bio-fungicides to control a range of economically important soil-borne fungal plant pathogens. Soils contain a large number and great diversity of oospore parasites, which may have the potential to reduce populations of plant pathogenic Phycomycetes in soil.

Predation

Predation is an association in which predator organism directly feed on and kills the pray organism. Predators may or may not kill their prey prior to feeding on them, but the act of predation often results in the death of its prey and the eventual absorption of the prey's tissue through consumption.

Many species of the soil-dwelling myxobacteria are predators of other microbes. Many myxobacteria, e.g.,Myxococcus xanthus, exhibit several complex social traits, including fruiting body formation and spore formation cooperative swarming with two motility systems, and group predation on both bacteria and fungi. Myxobacteria use gliding motility to search the soil matrix for prey and produce a wide range of antibiotics and lytic compounds that kill and decompose prey cells and break down complex polymers, thereby releasing substrates for growth. The nematophagous fungi are the best predatory soil fungi. Species ofArthrobotrytisandDactylellaare known as nematode-trapping fungi.

References

Atlas, RM and R Bartha. Microbial Ecology:Fundamentals and Applications. The Benjamin Cummins Publication co. Inc., 1998.

Gordis, L. Epidemiology. third edition. 2004.

Maier, RM, IL Pepper and CP Gerba. Environmental Microbiology. Academic press Elsevier Publication, 2006.

Park, K. Park's Text Book of social and prevention Medicine. 18th edition. 2008.