Basic Definition in the Study of Microbial Ecology:

1. Ecosystem: This is the combination of biotic and abiotic components of the specific environment. Usually, such a unit has the collection of organisms and abiotic components that are unique to it and as such, one eco-system is different from other.

2. Community: This is the collection of microorganism inhabiting the given site in ecosystem (i.e. ecosystem without environmental factors or abiotic factors).

3. Population: This is individual microbial species. Unlike community, it is more homogenous and specific.

4. Individual: This refers to individual organisms in population. Every ecosystem has, related with it certain physical, chemical and biological characteristics. Such factors administer the composition of community by dictating which of the individual microorganisms will be effectively established, and out of those established, few will be dominant while others are restricted and other groups are completely eliminated.

Thus, selection by environmental factors is significant. Those organisms which are established are those which are better adapted to abiotic conditions (water activity, PH, moisture, temperature, oxygen concentration etc) in that specific environment.

Organisms which can't cope with prevailing conditions are eliminated. Environment builds community through selection. It is as a result of this relationship with the community which we have certain Species of micro-organism as dominant.

5. The Habitat: A habitat is an area having the degree of uniformity in terms of abiotic components. They are thus also considered to be of ecological significance like surfaces of plants and animals, blood, soil, open sea, air, nasal passages, alimentary canal and so on. Size of the habitat is not significant as it differs considerably. The significant factor is that certain sets of conditions are uniform to that area.

6. Niche: The ability of microorganisms to make use of resources available in the habitat is differed and this has lead to idea of niche. Role of the particular organisms in the particular place is niche.

Microorganism could be divided in kinds depending on the functions in different habitats, those with narrow and those with broad range of tolerance. Those with narrow niche are highly particular and perform single function or role like obligate parasites, autotrophic organisms, while those that perform range of functions are said to have the broad niche like heterotrophic organism. Those which occupy narrow niche are merely eliminated if there is change in environmental conditions mostly as it affects their survival, but they although, flourish luxuriantly when their conditional requirements are met. Examples are photosynthetic micro-organisms which are negatively affected when source of light is blocked but they grow rapidly when there is light. On the other hand, those which have broad niche are not harshly affected by changes in environmental conditions.

Colonization and succession:

When the area is denuded or freshly exposed like when the tissue is freshly exposed or wounded or when we have earth quake that exposes earth's surface, a number of microorganisms will be deposited on such surfaces. First microorganisms to arrive on such surfaces are known as pioneers. These organisms grow and multiply to form pioneer community and from time the exposed areas are occupied by microorganisms, it is said to be colonized.

After establishment of the pioneer community, they feed on substrates, produced byproducts and other waste materials or metabolites and so environment becomes altered. Modified environment paves way for colonization by other organisms, whereas the pioneer communities are gradually eliminated.

Climax Community:

When colonization has occurred, succession follows and there is continuous modification of environment. Though, at a stage the community remains constant for at least some time and procedure of succession is stopped; and the community that characteristics that habitat results. This is known as climax community. Species composition at this phase is maintained reasonably constant for the period of time. Stability in composition doesn't mean that organisms don't die but number of organisms dying is rapidly replaced and this is the reflection of dynamic equilibrium. Climax community thus is a self-replicating entity where microorganisms and physical environment are in constant equilibrium.

Frequently, there are alternations in physical conditions e.g. when the large quantity of pollutants is introduced into the stream, organic matter component is eradicated and climax community is distorted. Though, such disturbances are temporary and original climax community is restored with time or as soon as disturbance is removed.

Succession in natural habitats:

Succession of Micro-organisms on Cellophane Film:

Tribe (1960) studied colonization and breakdown of cellophane film (pure regenerated cellulose). The breakdown of cellophane buried in the number of soils was studied. Although variations existed in soil kind, a general pattern of colonization was seen.

1. First colonizers were fungi and out of these, based on morphological and vegetative characters, three classes were recognized.

a) Those classified by coarse mycelium that ramifies through surface of cellophane and quickly starts decomposition by lysis of cellulose adjacent to their hyphae. Such fungi are generally in form genus Rhizoctonia and were evidently cellulolytic and dominant species at that phase.

b) Members of second group are generally species of the well known cellulolytic genera e.g. Chactomium species and Humicola species that exist as co-dominants. These fungi didn't ramify widely with cellophane but rather they penetrated the thickness at scattered sites using "rooting" hyphae which then branches to form the type of circular hyphal system.

c) Third group comprises of chytrids that develop on cellophane pieces. At this phase, cellophanes are no longer intact but in pieces.

Presence and activities of initial colonizers (pioneer community) were clearly associated to their ability to generate "cellulases". Though, other factors should have been involved as soil has other cellulolytic organisms that hardly ever appear on cellophane. Bacteria are fairly uncommon during the initial stage of fungal attack. During mycelium senescence though they rapidly increase and apparently use either material diffusing from hyphae or hyphae itself.

2. Second colonizers are bacteria. In turn bacteria support the population of nematodes and protozoa. After micro-organisms have colonized the cellophane/cellulose, mites, springtails and other worms turn out to be active and substrate becomes unidentifiable by the passage through guts of these organisms. Cellophane decomposition thus involves the wide range of micro-organisms and small animals whose occurrence depends on nutrient availability.

Succession of micro-organisms on dung:

If the fresh dung is placed under the bell jar and a moderately appropriate humidity is kept, a succession of fungal fructifications can be seen. The first to appear are Zygomycetes followed by Ascomycetes and then Basidiomycetes. Every group has been considered to represent sugar, cellulose and lignin utilizers respectively.

Harper and Webster (1964) while confirming sequence have illustrated that it is not the succession based on nutritional factors. They grew number of fungi involved under the variety of conditions and illustrated that each group had the characteristic minimum time before commencement of development and appearance of fruiting bodies. If fungi are listed in order based on minimum time, the sequences is also determined to be the same as on dung. According to the authors, succession of fruiting bodies on dung is associated with duration of essential developmental periods rather than dissimilar assimilatory abilities.

Colonization of sterile human hair:

Griffin (1960) made the sequential study of sterile human hair placed on surface of different soils. In general, his observations illustrated that first colonizers where Fusarium species, Penicillium species and some Mucorales. These species depend on simple sugars. Second group of colonizers comprises Chaetomium cochloides, Humicola species, Gliocladium roseum and Penicillium species. First two are cellulolytic whereas last two are polysaccharide users. Third group are keratinolytic members of Gymnoascaccae that are mostly Ascomycetes.

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