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Microbial Growth at Extreme Temperature

Microbial Growth at Extreme Temperature
Fig: microorganisms (source: google)
1    Microbial growth at high temperature
There are many examples of environments with extreme temperature. The environment with high temperature includes both terrestrial and submarine environment. Microbial life flourishes in this high-temperature environment. Two types of organisms: Thermophiles (whose optimum growth temperature is about 45) and Hyperthermophiles (whose optimum growth temperature is 80) are predominant in these environments.
Examples of thermophiles and hyperthermophiles
Optimal temperature
Methanosarcina thermophila
Methanobacterium wolfei
Methanobacterium thermoautotrophicum
Archaeoglobus profundus
Thermoproteus uzoniensis
Staphylothermus marinus
Pyrodictium abyssi
Pyrococcus furiosus, Hyperthermus butylicus
Pyrodictium occultum, Pyrococcus woesei
Habitats for thermophiles and hyperthermophiles
·         Thermal vents
·         Hot springs
·         Boiling steam vents
Molecular adaptation
Many mechanisms allow microorganisms to survive at temperature that would normally denature proteins, cell membranes and even genetic material DNA.
1  First, in terms of proteins/enzymes
The enzymes and the other proteins in these organisms are much more stable to heat than those mesophiles. It appears that a critical amino acid substitution in one or few locations in these enzymes allows it to fold in such a way that is consistent with heat stability. Stability of protein in hyperthermophiles is also improved as a result of an increased number of salt bridges (cations that bridge charges between amino acid residues). These bridges help proteins to remain folded even at high temperature. Finally, it appears that certain solutes such as di-inositol phosphate and monosylglycerate are produced in significant quantities and helps to stabilize proteins against thermal degradation.
1Second, in terms of cell membrane
In addition to enzymes and other components of cells, the cytoplasmic membrane of thermophiles and hyperthermophiles needs to be stable. Thermophiles typically have lipids rich in saturated fatty acids thus allowing the membrane to remain stable and functional at high temperature. Saturated fatty acids form a stronger hydrophobic environment than do unsaturated fatty acids which help in membrane stability. Hyperthermophiles, most of which are Archaebacteria, do not contain fatty acid at all in their membrane but instead they possess 40 carbon chain hydrocarbon components composed of repeating units of 5 carbon compounds- Isoprene. Isoprenes bonded by ether linkage to glycerol phosphate provide stability to membrane. In addition, overall structures of these membranes form monolayer which is much more heat resistant than lipid bilayer of bacteria and eukarya.
    Third, in terms of nucleic acid, DNA
Thermophiles contain special DNA binding proteins that arrange the DNA into globular particles that are more resistant to melting. Another factor that is common to all hyperthermophiles is a unique DNA gyrase enzyme. This DNA gyrase acts to introduce positive super coils in DNA, providing considerable heat stability.
2. Microbial growth at low temperature
Much of the Earth surfaces experience fairly low temperature.the oceans have an average temperature of 5. Vast land areas of Antarctic region are permanently frozen. These cold environments are not sterile some microorganisms can be found alive and growing. Even in many frozen materials, there are usually microscopic pockets of liquid water present where microorganisms can grow. Some of the best studied psychrophiles are algae. The most common snow algae is Chlamydomonas nivalis. Some other examples of pyschrophiles include: Psychrobacter, member of halomonas; Pseudomonas spp., Hyphomonas spp., Sphingomonas spp., Chryseobacterium greenlandensis, etc.
Common habitats of Psychrophiles:
·         Alpine and Arctic soil
·         High latitude and deep ocean
·         Polar ice, glaciers, snow fields, etc.
Molecular adaptation to Pyschrophiles
Psychrophiles produce enzymes that functions optimally in the cold. The molecular basis for this is not entirely understood but it has been observed that an average cold active enzyme has greater amount of α-helix and lesser amount of β-sheet, secondary structure than enzymes that are inactive in cold. Because these β-sheets tend to form more rigid structures, greater α-helix content of cold active enzymes allows these proteins greater flexibility in the cold temperature.
Another feature of Psychrophiles is that their cytoplasmic membranes contain higher amount of unsaturated fatty acids which helps to maintain a semi-fluid state of the membrane at low temperature. The lipids of some Psychrophiles also contain polyunsaturated fatty acids and long chain hydrocarbons with multiple double bonds.

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