Microbial Growth at Extreme Temperature

Microbes in Extreme Environments

Various abiotic factors strongly influence the ecological distribution and functioning of a microbial population. Nutritional constraints and environmental tolerance regulate or exclude the existence of microorganisms in a various environment according to “Liebig’s Law of Minimum” and “Shelford’s Law of Tolerance” respectively.

Liebig’s Law of Minimum

Liebig, a German agricultural chemist, recognized that like atoms in a molecule, elements in living organisms are present in distinct proportion. According to him, the total yield or biomass of any organisms will be determined by the nutrient present in a minimum concentration in relation to the other requirements of that organisms.

Shelford’s Law of Tolerance

The occurrence and abundance of organisms in the environment are determined not only by nutrients but also by various physiological factors such as temperature, p^H, salinity and many others. Shelford’s law says that there are many environmental factors and boundaries of those environmental factors govern the survival of the organisms. For an organism to succeed in a given environment, each of these condition must remain in the tolerance range of that organism. If any conditions such as temperature exceed the minimum or maximum tolerance, the organism will fail to thrive and will be eliminated. Tolerance range for a given organism for a given parameter are however, somewhat interactive with other parameters. Thus a microorganism that is not able to survive at a particular temperature in an ecosystem with a particular H⁺ concentration.

Microbial Growth at Extreme Temperature

1. Microbial growth at high temperature

There are many examples of environments with extreme temperature. Environment with high temperature includes both terrestrial and sub-marine 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

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.

1. Second, 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.

1. Third, in terms of nuclei 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.

Reference

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.