An extremophile is an organism that prospers in extreme physical or geochemically extreme conditions. The word extremophile comes from the Latin extremus meaning “extreme” and Greek philiā meaning “love.” 1) On the other hand, mesophiles or neutrophiles, are organisms that live in moderate environments. Most extremophiles known today are microbes but they are present in various lineages of bacteria and archaeans. However, not all archaeans are extremophiles, some are mesophilic, organisms that grow best in moderate temperature. 2)
In the 1980s and 1990s, it was discovered that microbial life was able to adapt and survive in extreme environments, unlike complex organisms. Some scientists hypothesized that life on Earth may have begun in hydrothermal vents under the surface of the ocean. Dr. Steinn Sigudsson, an astrophysicist, said, “There are viable bacterial spores that have been found that are 40 million years old on Earth - and we know they're very hardened to radiation.” 3)
There are many different types of extremophiles, many of them belonging to multiple categories. Some examples of extremophiles include Halophile and Arsenophile.4)
Halophiles are a type of extremophile that prosper in environments with high concentrations of salt. The word halophile comes from the Greek for “salt-loving.” Halophiles can be classified as Archaea, bacteria, and even as eukaryota. Some halophile species give off a red color from carotenoid compounds. These species have a photosynthetic pigment called bacteriorhodopsin. 5) This pigment moves protons across the membrane and out of the cell by capturing light energy. 6)
Halophiles are classified as slight, moderate or extreme depending on their halotolerance 7) or their ability to adapt to environments of high salinity. 8) They can be found in places where the concentration of salt is five times greater than the salt concentration of the ocean; places like the Great Salt Lake in Utah, the Dead Sea, and in evaporation ponds.
Few organisms have been able to adapt to an environment of high salt concentration. Halophilic organisms burn up energy to exclude salt from their cytoplasm to avoid protein aggregation. To survive these environments, halophiles increase internal osmolarity of the cell to prevent desiccation. This is done in one of two ways. In the first, bacteria, some archaea, yeasts, algae and fungi, accumulate organic compounds in the cytoplasm. Some common compounds include amino acids, sugars, polyols, betaines and ectoines. The second way to increase internal osmolarity involves the influx of potassium ions into the cytoplasm. The second method is only used by the Halanerobiales, Halobacteriaceae and Salinibacter ruber Families.
The extreme halophiles or haloarchaea require at least a 2 M salt concentration and are found in saturated solutions. If these organisms are exposed to anything other than a high salt environment, they will perish because they require salt for growth. Many cells immediately lyse when they are placed in distilled water from the change in osmotic state.
The genomic and proteomic analysis showed distinct molecular signatures for environmental adaptation of halophiles. At the protein level, halophilic species are characterized by low hydrophobicity, overrepresentation of acidic residues, underrepresentation of Cys, lower tendencies for helix formation, and higher tendencies for coil structure. Also, the core of these proteins is less hydrophobic and at DNA level, halophiles have distinct dinucleotide and codon usage. 9)
Arsenophiles are organisms that prosper in environments that are high in arsenic content, using it as a source of energy for growth. The key to generating energy from arsenic is in its redox chemistry, which is characterized by at least four oxidation states: -3, 0, +3, and +5. Microbial life is supported by the energy produced from the oxidation of As3+ to As5+. The electrons from this transformation are transferred to nitrate or oxygen, an electron acceptor. 10)Some organisms have developed resistance mechanisms for arsenic but others require arsenic for their physiology. Arsenic is very closely related to phosphate in its chemical properties; this allows arsenic to enter the cell through existing phosphate transporters. 11)
Arsenic is known to sorb to mineral phases, which can impact its environmental mobility. Arsenite, on the other hand, sorbs less strongly to some key mineral phases. This might result in the mobilization of the more mobile arsenite, also more toxic than arsenate. 12) Here is the arsenic geocycle: 13)
For many years it has been known that arsenic is poisonous but the mode of toxicity depends on the chemical form of the metalloid. Microorganisms have dynamic mechanisms for facing the toxicity of arsenic in the environment. 14) Arsenate imitates phosphate and can enter the cell through transporters meant for phosphate. Once arsenate is in the cell, it interferes with phosphate-based energy-generating processes and can inhibit oxidative phosphorylation. Arsenite enters differently and binds to the thiol groups such as pyruvate dehydrogenase.
Microorganisms have different methods to protect themselves from arsenic. Fungi, for example, uses methylation to produce monomethylarsonic acid (MMA) or dimethylarsinic acid (DMA). Prokaryotes can produce methylated arsines that remove arsenic from the local environment by forming it into a gaseous compound. Bacteria and yeast use a different methodbased upon the “ArsC” arsenate reductase protein. The gene for this enzyme is encoded on plasmids. ArsC is found in the cytoplasm of the cell and mediates the reduction of arsenate to arsenite. Arsenite, being toxic, is then excreted by an ATP-dependent efflux pump, ArsB. Finally, to close the arsenic cycle, arsenite-oxidising bacteria couple the oxidation of As3+ to the reduction of molecular oxygen or nitrate. 15)
The ars operon is the most important method of arsenic tolerance in bacteria. The ars genes are found co-transcribed by a large variety of genomic configurations arranged in a way that depends on the specific strain.16)
Studying environments rich in arsenic helps us gain more of an understanding about the biogeochemical cycle of arsenic. Two locations in California where the arsenic levels are pretty high are Mono Lake and Searles Lake. The arsenic comes from the inflow of hydrothermal water. These lakes have no outflow so the arsenic that is transported there remains for many, many years. An extremely halophilic arsenate-respiring bacterium called SLAS-1 was isolated from Searles Lake and was capable of growth at salt-saturation. 17)