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Extremophiles are organisms that live in conditions which humans consider “extreme.” “Extreme” environments include but are not limited to extreme pressure, extreme cold, intense heat, highly acidic environments, and highly saline environments. These conditions were once believed to not have the ability to sustain life.
There are three domains of life: Eukarya, Bacteria, and Archaea. Each of these domains share features with the other while having their own unique set of characteristics, and none of these domains are ancestral to the others. The most notable extremophiles belong to the Archaean domain. Even though penguins are classified as extremophiles, most known extremophiles are micro-organisms; the main types of extremophiles that scientists study are from the Archaea and Bacteria domain of life.
Studying extremophiles can offer us a solid grasp of the physiochemical limitations defining life on our planet. It is hypothesized that primitive Earth environments were abundant in extreme conditions – most of these environments were extremely hot. This leads to the idea that extremophiles are vestiges of ancient organisms and may provide an understanding of how life on Earth emerged.
Extremophiles owe most of their ability to be able to sustain themselves in such harsh conditions to proteins. Protein folding is an essential part of surviving in all living organisms – they are needed to all living cells to grow, function and repair. RNA translation is a required step in the process of making proteins – without translation, organisms would have no other possible way to make proteins and thus function. There is no one fundamental set of adaptions the fits every environment. Instead, Archaea have evolved separate protein functions to survive specific environments. By understanding how protein adaptations allow organisms to survive in extreme environments, we hope to be able to understand the limitations of life not only on our planet Earth, but on other places on our Solar System.
One type of extremophiles is called Psychrophiles. These organisms are able to survive at very low temperatures. These organisms are found in areas that are perpetually cold, for example the deep sea, permafrost, glaciers, snowfields and the polar regions. Deep ocean water is a fairly stable temperature at approximately 2°C. However, the salt content of the water in colder regions of ocean water allow the water to reach temperatures as low as -12°C without freezing up. In fact, microbial activity has been detected in soils that have been frozen below -39°C.
Genomic (from study of genes), proteomic (from study of proteins), and transcriptomic (from study of the transcriptome, or gene expression at specific circumstances) studies suggest that Psychrophiles have various features that allow them to translate RNA and perform protein folding in cold conditions.
In normal conditions, proteins, namely enzymes, lose activity as temperatures drop below 20°C, which is not a good situation for a cell if it needs to grow. Enzyme activity declines at low temperatures because the average kinetic energy in the cell is low. Low kinetic energy means that conformational movements become slower and consequently, less efficient.
Psychrophilic proteins are more flexible so therefore they are better able to move and change conformation. This means that psychrophilic proteins can maintain high activity even at low temperatures. On top of that, a psychrophilic enzyme has typically 10 times more activity than a mesophilic (normal temperature) enzyme.
Thermophiles are able to grow between 50°C and 70°C, while hyperthermophiles can grow optimally up to 105°C, with a limit of 110 °C to 121 °C. These organisms can be found terrestrial geothermally-heated and marine habitats including sediment of volcanic islands, hydrothermal vent systems, shallow terrestrial hot springs, and deep sea hydrothermal vents.
All cells have an outer membrane that regulates what comes in and out of the cell. The cell membrane also serves to protects the inner contents of the cell from the environment. A universal component of the cell membrane is the lipid bilayer, which provides the barrier in the membrane. Since lipids are fats, they are insoluble in water. The most common class of lipid molecule found in the bilayer is phospholipids.
In extremely hot conditions, the cell membranes of “normal” organisms will be more flexible – when the membrane is more flexible it may lead to cell lysing – this causes the membrane to break, and the cell will not be able to protect itself and it will die. Another fate for “normal” proteins in extreme heat is that they can undergo irreversible unfolding, exposing the hydroponics cores, which causes aggregation. When proteins form aggregates, they will not function properly anymore.
In thermophiles, however, the phospholipids have some adaptations. The fatty acids of the phospholipids are longer, and have more side chains, and saturated. The increased number of large hydrophobic residues, disulphide bond, and ionic interactions promote thermostability. Better backing of thermophiles would prevent water molecules from penetrating inside and destabilising the protein core (water destabilises proteins due to its efficiency in hydrogen bonding with the macromolecule). This provides a rigid membrane, giving it a stable membrane in a hot environment.
To avoid denaturation and forming aggregates, the thermophile can form heat shock proteins. When these proteins are formed, they can protect the protein from forming aggregates, they can also refold the protein structure, which may allow the protein to function in the cell.
Halophiles are salt-loving organisms that thrive in saline environments. These organisms can be found in hypersaline environments all over the wold in underground salt mines, coastal and deep-sea locations, and artificial salterns. The Dead Sea and the Great Salt Lake, which are extremely salty environments are notable examples of where halophilic organisms can be found. Sodium chloride is capable of altering the conformation, stability, and solubility of a protein, consequently affecting the protein’s ability to function.
Some halophilic bacteria and eukaryotes are able to prevent the entry of salts into the cell and synthesise small organic molecules, known as osmolytes, to balance the osmotic pressure that is generated when you have a region of higher solute concentration compared to the other regions.
Halophilic Archaea, however, take in high concentrations of salts. For a non-halophillic organism, when the salt concentrations are high, water tends to surround the ionic lattice of the salt. Therefore, there is less water available for the proteins of the non-halophilic organism. The reduced readiness of water causes the hydrophobic amino acids in a non-halophilic protein to lose hydration and aggregate. This disrupts the normal structure of proteins that non-halophilic proteins are used to – the stability of the organism is altered.
Proteins in halophilic archaea have adaptions that allow them to take advantage of the high concentrations of inorganic salt to stabilise their native protein fold. Halophilic proteins have large increase in acid residues on the protein’s surface. There are two main possible roles that these acid residues are thought to have. The first role is to keep the protein remained within solution. The acid residues can cause the surface of the protein to be more negatively charged. Water molecules will be attracted to the negative charge, allowing the protein to compete with other ions for the water molecules. The acid residues can also bind with hydrated cation, which can maintain a shield of hydration around the protein.
Some extremophiles are adapted simultaneously to multiple stresses (polyextremophile); common examples include thermoacidophiles and haloalkaliphiles. These organisms can tolerate two or more extreme environmental factors.
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