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The microorganisms depending upon their needs to grow in their natural homes or ecological niche may be grouped into the following broad categories keeping in view their requirement for oxygen, source of energy (carbon) and temperature.

(A) On basis of their oxygen requirement they are:

(i) Aerobes or aerobic microorganisms which have capability to grow on media or substrates in contact with air, e.g. some species of Bacillus and Thermoactinomyces.

(ii) Anaerobes or anaerobic microorganisms which have capability to grow in absence of oxygen, e.g. species of Bactervides, Clostridium, Selenomonas, Succinivibrio and Veillonella.

(iii) Faculative anaerobes or facultative anaerobic microorganisms that is they are optional in their habit and have ability to adopt an alternative life style. They are not strictly anaerobes and can grow in absence or presence of air (Oxygen).

(B) On the basis of their nutritional requirement or carbon source they are grouped as:

(i) Autotrophs or microorganisms which use carbon dioxide for most or all of their carbon requirement. All obligate autotrophs appear to be either chemolithotrophs or photolithotrophs. In many autotrophs CO2 fixation occurs via the Calvin cycle or via the reductive tricarboxylic cycle. Some acetogens fix CO2 via a different
pathway sometime called the activated acetic acid pathway in which acetyl COA can be synthesized by the reduction of two molecules of CO2. A similar pathway occurs in Desulfovibrio in which the methyl and carboxyl groups of acetyl CoA are derived from formate and CO2 respectively. An analogous (similar or parallel)
pathway occurs in methanogens.

(ii) Heterotrophs are the organisms which utilise organic compounds for most or all of its carbon requirements. The terms heterotroph is often used for Chemoorganotrophs. Although chemoorganotrophs and phototrophs may also be heterotrophic.

(C) On the basis of temperature requirement the microorganisms are grouped as:

(i) Psychrophiles, which are cold loving and grow optimally at or below 15°C and have upper limit for growth about 20°C and lower limit of growth 0°C or below.

(ii) Mesophiles are the organisms whose optimum growth temperature lies within range generally accepted as 20 — 45° C.

(iii) Thermophiles or heat loving microorganisms grow optimally above 45°C e.g., Methanococcus, Pvrodictium, Sulpholobus. Thermobacteroides, Thermophilum, Thermomicrobium, Thermoplasma, Thermoproteus, Thermothrix and Thermus. Many published ad hoc definitions of thermophile are mutually incompatible often because a given definition may be applicable is only a limited context, moreover, such definitions are frequently incompatible with the generally accepted upper limit of ‘mesophile’.

(iv) Hyperthermophiles and so called superhyperthermophiles are the heat loving microorganisms which optimally grow above 80 to 100°C, e.g. Thermus aquatics from which enzyme Taq polymerase have been isolated, Pyrococcus furiousus is an another microorganism from which the Pfu polymerase has been isolated. These enzymes are used in PCR technology. Pyrolobus fumari occur in the walls of smokers and Methanopyrus in deep sea chimneys and produce methane. Some such chimneys collected from, bottom of seas have been displayed in National Institute of Oceanography at Vasco in Goa in India.

(D) On the basis of their habitat microorganisms have been grouped as:

(i) Soil microorganisms which occur in soil, e.g., some actinomycetes and fungi.

(ii) Aquatic microorganisms found in water e.g., some bacterial species and aquatic fungi.

(iii) Aeromicroflora comprise the microorganims growing in air.

(E) On the basis of their nutritional habit they are grouped as:

(i) Saprophytes which get their nutrition from dead organic organisms.

(ii) Parasites, which obtain their nutrition form a live plant or animal.

(iii) Symbionts are the microorganisms where both host and microorganisms are benefitted.

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