Microorganisms are an important component of an ecosystem. Several ecological categories can be made on certain grounds while grouping the microorganisms. They are: 1. Based on Oxygen Requirement 2. Based on Carbon Sources as Energy 3. Based on Temperature 4. Based on Habitat 5. Microorganisms Living in Extreme Environment Extremophiles 6. On the Basis of Mode of Nutrition or Habit.
Contents
1. Based on Oxygen Requirement:
On the basis of their ability to grow in the presence or absence of O2 they can be divided into three distinct categories: aerobes (which grow in the presence of O2), anaerobes (which grow only in the absence of O2), and facultative anaerobes (which has capability to grow either in the absence or the presence of the air).
2. Based on Carbon Sources as Energy:
Carbon is a chief source of energy and a building block of cell wall and other cell components.
All living organisms are divided into the two major groups, the autotrophs and the heterotrophs:
(i) Autotrophs (or Lithotrophs):
In autotrophs the carbon source is CO2. Energy is provided through photosynthesis in photoautotrophs (i.e. phototrophs) or oxidation of inorganic compounds as in chemotrophs. Examples of autotrophs are cyanobacteria, microscopic algae, and photosynthetic bacteria.
(ii) Heterotrophs (or Chemoorganotrophs):
In heterotrophs the sources of carbon and energy of biosynthetic mechanism of the cell are the dead organic materials. These materials are broken down by the heterotrophs for their carbon and energy to form their cell components and cell as well. There is a large number of heterotrophs represented by bacteria, fungi, actinomycetes, etc.
3. Based on Temperature:
When temperature becomes a limiting factor i.e. governs the growth and activities of microorganisms, the latter becomes adapted to the new environment.
On the basis of temperature regime, microorganisms may be categorised in the following groups:
(i) Psychrophiles:
Psychrophiles are those microorganisms which grow at a very low temperature (below 20°C).
Further the microorganisms growing at low temperature can be classified into two groups:
(a) Facultative psychrophiles, and
(b) Psychrotrophs.
Facultative psychrophiles are capable of growing at temperature ranging from 0°C or less to 20°C with an optimum growth at 15°C.
Some isolates from Antarctic waters have been found to show maximal growth at 10°C or less. On the other hand psychotrophs are those microorganisms which can grow at 5°C or below, irrespective of their upper or optimum growth temperatures. Singh (1984) have reviewed the ecological aspects of microbial life at low temperatures.
In the troposphere (upto 27,000 meter altitude) where temperature remains below -40°C, bacteria, fungi and other microorganisms have been isolated. Procter and Parken (1942) have recorded the presence of Bacillus over polar Pacific and Atlantic oceans where the temperature remains below -20°C.
Moreover, from some glacial caves (e.g. caves in the Arctic, Lapland the Pyrenees, the Alps in Romania) where temperature ranges between -0.8°C and 5°C, certain bacteria such as Arthrobacter, Pseudomonas and Flavobacterium have been isolated. The optimal temperature required for growth by Polaromonas vacuolata is 4°C.
(ii) Mesophiles:
The microorganisms growing optimally at temperatures between 25°C and 35°C, and have capacity to grow at about 15°C to 45°C are known as mesophiles.
Most of microorganisms are mesophiles and constitute the major component of an ecosystem, for example cyanobacteria (e.g. Nostoc, Anabaena, Oscillatoria, etc.), bacteria (e.g. Rhizobium, Mycobacterium, Corynebacterium, Azotobacter, etc.), fungi (e.g. Fusarium, Trichoderma, Puccinia, Pythium, etc.), amoebae, etc.
(iii) Thermophiles (Heat-loving Microorganisms):
Microorganisms growing readily at temperatures of 45 to 65°C are known as thermophiles. The obligate thermophiles are incapable of multiplying below 40°C. Johri and Satyanarayana (1984) reviewed thermophilic fungi of paddy straw (e.g. Aspergillus fumigatus, Chaetomium thermophila, Humicola lanuginosa, and Thermoascus aurantiacus), coal mine soil (e.g. A. fumigatus, C. thermophila, Torula thermophile, Paecilomyces sp., etc.)
(iv) Hyperthermophiles and Super-hyperthermophiles:
The microorganisms that prefer to grow above 80°C to 100°C (the boiling point of water at sea level) are called hyperthermophiles. Thomas D. Brock of the University of Wisconsin about 30 years ago first discovered the earliest specimens of microbial life in hot springs and other waters. In 1960 Brock with his colleagues identified the first hyperthermophile capable of growing at temperature greater than 70°C.
The bacterium is now known as Thermus aquaticus. From this bacterium an enzyme named Taq has been isolated which is widely used in polymerase chain reaction (PGR) technology. Some users of PGR have replaced the Taq with Pfu polymerase. This enzyme is isolated’ from Pyrococcus furiosus.
In addition, this team found the first hyperthermophile in an extremely hot and acidic spring which is known as Sulfolobus acidocaldarius. This bacterium grows prolifically at temperatures as high as 85°C. To date more than 50 hyperthermophiles have been isolated, many by Karl O.
Stetter and his colleagues at the university of Regensburg in Germany. The most heat – resistant of these microbes is Pyrolobus fumarii which grows in the walls of smokers. It multiplies at temperature upto 113°C. It stops growing at temperatures below 90°C. Example of the other hyperthermophile is Methanopyrus that lives in deep-sea chimneys and produces methane.
Now there is a question about the upper limit for life. Do super-hyperthermophiles capable of growing at 200 or 300°C exist? No body knows, although current understanding suggests that the limit will be about 150°C.
Above this temperature probably no life forms could prevent dissolution of the chemical bonds that maintain the integrity of DNA and the other essential molecules. All such organisms growing at extremes of environment are termed as extremophiles.
4. Based on Habitat:
On the basis of habitat in broad sense, microorganisms are divided into the following groups:
(i) Soil Microorganisms:
Microorganisms dwelling in soil subsystem are known as soil microorganisms or soil microflora, and the branch of microbiology dealing with soil microflora is known as soil microbiology.
(ii) Aquatic Microorganisms:
Microorganisms residing in water subsystem irrespective of its quality or physical or chemical nature are in broad sense called aquatic (water) microorganisms. The branch of microbiology dealing with aquatic microorganisms is known as aquatic (water) microbiology. Most of the human pathogens spread through water.
(iii) Aeromicroflora:
Microorganisms living in air are generally known as aeromicroflora or microorganisms of air, and the branch of microbiology dealing with aeromicroflora is known as aeromicrobiology. Several fungal and viral diseases are transmitted through air.
5. Microorganisms Living in Extreme Environment Extremophiles:
Extremophiles thrive under conditions that would kill the other creatures. The molecules that enable extremophiles to prosper are becoming commercially useful now a days. Of the particular interest are the enzymes that help the extremophiles to function in brutal circumstances. Psychrophiles and hyperthermophiles have been described earlier.
Example of other groups of high and low pH loving microorganisms are also not uncommon. Most natural environments on the earth are essentially neutral, having pH values between 5 and 9. Acidophiles (acid loving) thrive in the rare habitats having a pH below 5, and alkaliphiles favour habitats with a pH above 9.
6. On the Basis of Mode of Nutrition or Habit:
There are different ways by which the microorganisms derive nutrition for their growth and development:
(i) Saprophytism:
Saprophytism is a phenomenon which refers to getting nutrition from dead organic materials, and such microorganisms are known as saprophytes for example Aspergillus, Penicillium, Rhizopus, Mucor, etc. The saprophytes are equipped with extracellular enzyme- producing capacity according to the available substrate.
Sometime in the presence of a living host a few saprophytes change their tendency and cause disease. Such saprophytes are called facultative parasites. Facultative parasites are basically saprophytes but have tendency to behave as parasite as well. Such microorganisms are very dangerous. They are also termed as opportunistic microorganisms. For example, some species of Fusarium, Pythium, etc. are facultative parasites.
(ii) Parasitism:
Parasitism refers to deriving nutrition from a living plant or animal host, and microorganisms associated with parasitism are known as parasites (living upon). There are certain nutrients which are not found in dead organic materials.
For such nutrients the parasites have to infect the plant or animals. On the other hand, it can be said that parasitism is a tendency of parasites to infect living hosts. When a parasite is very virulent and cannot live without a living host, it is called obligate parasite such as Puccinia, powdery mildews, etc. Obligate parasitism is the highest level of specialization for deriving nutrition.
On the other hand a parasite, in the absence of a suitable living host can pass its life as saprophyte. It is a second mode of leading the life and survival mechanism.
Such types of parasites are known as facultative saprophytes i.e. the parasites that have faculty to live as saprophyte in the absence of a suitable host, for example Phytophthora infestans (causing late blight of potato can be cultured in laboratory on oat meal agar medium), Taphrina deformans (a leaf curl fungus of peach, etc.), some smut fungi, etc.
(iii) Symbiosis:
In parasitism advantages are only to the microorganisms. The hosts are the losers. Consequently, there develops disease. However, in other case both the microbes and hosts are benefited as far as nutrition is concerned.
Such association of mutual benefit is known as symbiosis. Symbiosis can be seen in lichens, mycorrhiza, root nodules of legumes and non-leguminous plants.