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Microbial Growth at Extreme Salinity and Extreme pH

Microbial Growth at Extreme Salinity
Microorganisms that tolerate or require high salt concentration are called halotolerant and halophilic organisms respectively. The growth of halophiles, therefore, requires at least some NaCl but the optimum varies with the organisms. The terms mild halophile and moderate halophile are used to describe halophiles with low (1-6%) and moderate (6-15%) NaCl requirements respectively. Extreme halophiles are those which require 15-30% of NaCl.
In nature, there are such many environments where the salt concentration is found to be maximum. One of the best examples of this type of environment is the salt lake. Another example is the dead sea (50-70%). At high salt concentration, the hypertonic environment tends to dehydrate non-halotolerant microorganisms. In addition affecting osmotic pressure, high salt concentration tends to denature proteins, that is they disrupt the tertiary structure of proteins which is essential for enzymatic activity.
Examples of halophilic bacteria include Halobacterium spp., Halococcus spp., Ectothiorhodospira spp.
Examples of halotolerant bacteria include Staphylococcus spp., Halomonas spp.
Molecular adaptation of halophiles
Many microorganisms that survive these environments of halophilic utilize several mechanisms. The main mechanism of salt concentration of a balancing solute to equal the salt concentration found external to the cell. When an organism grows in a medium with a low water activity, it can obtain water from its environment only by increasing its internal solute concentration. An increase in internal solute concentration can be accomplished by either pumping inorganic ions into the cell from the environment or synthesizing on concentrating organic solutes. The solute used inside the cell for adjustment of cytoplasmic water activity must be non-inhibitory to biochemical processes within the cell. Such compounds which are either synthesized or concentrated inside the cells are called compatible solutes. These substances are highly water soluble sugars, alcohol, amino acids or their derivatives or in case of extremely halophilic bacteria, Kions. The amount of compatible solute that can be made or that can be accumulated is a genetically directed characteristic. For instance, Gram positive cocci of the genus Staphylococcus, a halotolerant organism, can use the amino acid ‘proline’ as a compatible solute. Similarly, glycine betaine, a derivative of the amino acid ‘ glycine’ in which the protons on the amino group are replaced by three methyl groups which leave permanent positive charge on N-atom increasing its overall solubility, is widely distributed as a compatible solute in many halophilic bacteria. Halophilic green algae produce mainly glycerol as a compatible solute.
Microbial Growth at Extreme pH
Microorganisms generally cannot tolerate extreme pH values. The pH of an environment affects microorganisms and microbial enzymes directly and also influence the dissociation and solubility of many molecules that indirectly influence microorganisms. The pH determines in the impact the solubility of COinfluencing the rate of photosynthesis, the availability of required nutrients such as ammonium and phosphate and the mobility of heavy metals such as copper which are toxic to microorganisms. There are, however, acidophilic and alkaliphilic organisms that can tolerate or even require extreme pH for growth. Examples of acidic environment include acid hot spring, the GI tract, mining waste streams, acid mine waste water, etc. Some acid-tolerant bacteria like Lactobacillus
and acidophile like Thiobacillus, Sulfolobus create their own low pH environment by producing acids. Lactobacillus is a mixed acid fermenter and Sulfolobus produces sulfuric acid. Bacillus acidocaldarius and Thermoplasma acidophilus are heterotrophic thermoacidophiles that live in an acidic environment created by chemolithotrophic, sulfur oxidizer such as in acidic hot spring. Archae like Picrophilus are found in dry and extremely acidic soil (pH < 0.5) solfataric gases to about 55. Similarly, many bacteria and fungi can tolerate alkaline pH up to 9. True alkaliphiles include some Bacillus strains such as Bacillus alcalophilus and Bacillus pasteurii. Halophilic microorganisms such as HalobacteriumNatrobacterium and Natronococcus are also alcaliphiles and live in saline lake with high pHClostridium paradoxum has a pmaximum than 10 at 55.
Molecular adaption
A common feature of microorganisms that tolerate or even require pH extreme for growth is that their cytoplasm is maintained close to neutrality because, despite the requirements of a particular organisms for specific pH, the optimal growth pH represents the pof the extracellular environment only. The intracellular pH must remain near neutrality in order to prevent destruction of acid or alkali labile macromolecules in the cell. Furthermore, their cell wall and membrane need to be adapted to keep their integrity under the pH extreme and performed chemo-osmotic ATP synthesis under these unusual conditions. The precise structural and biochemical adaptation remain insufficient however; a common strategy used by microorganisms to deal with high or low pH values usually involves modification in the cell membrane. The first of these modifications is the structure of membrane compounds to allow them to be acid tolerant for acidophiles. This include the incorporation of very long chain dicarboxylic fatty acids (32-36 carbon) which make up more than 50% of the membrane fatty acid. These specialized fatty acids inhibit acid hydrolysis of the membrane.
The second modification involves control of ion transport across the membrane. But controlling ion transport, these organisms can maintain an internal pH in the range of 5-7, even though the external pH < 2.

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