Piezophile
Introduction. A piezophile (from Greek "piezo-" for pressure and "-phile" for loving) is an organism with optimal growth under high hydrostatic pressure or, more operationally, an organism that have its maximum rate of growth at a hydrostatic pressure equal or above 10 MPa (= 99 atm = 1,450 psi), when tested over all permissible temperatures. Originally, the term barophile was used for these organisms, but since the prefix "baro-" stands for weight, the term piezophile should be given preference. Like all definitions of extremophiles, the definition of piezophiles is anthropocentric, and humans consider that moderate values for hydrostatic pressure  are those around 1 atm (= 0.1 MPa = 14.7 psi). Hyperpiezophile is defined as an organism that have their maximum rate of growth above 50 MPa (= 493 atm = 7,252 psi). The current record for highest hydrostatic pressure where growth was observed is 130 MPa (= 1,283 atm = 18,855 psi), by the archaea "Thermococcus piezophilus". Obligate piezophiles refers to organisms that are unable to grow under lower hydrostatic pressures, such as 0.1 MPa. In contrast, piezotolerant organisms are those that have their maximum rate of growth at a hydrostatic pressure under 10 MPa, but that nevertheless are  able to grow at lower rates under higher hydrostatic pressures. Most of the Earth's biosphere (in terms of volume) is subject to high hydrostatic pressure, and the piezosphere comprises the deep sea (at the depth of 1,000 m and greater) plus the deep subsurface (which can extend up to 5,000 m beneath the seafloor or the continental surface). The deep sea has a mean temperature around 1 to 3 °C, and it is dominated by psychropiezophiles, in contrast to the deep subsurface and hydrothermal vents in the seafloor, which are dominated by thermopiezophiles that prosper on temperatures above  45 °C (113 °F). The high pressures experienced by these organisms can cause the normal fluid cell membrane to become waxy and relatively impermeable to nutrients. The high pressure decreases the ability of the subunits of multimeric proteins to interact. Thus, large protein complexes must interact to decrease pressure-related effects and regulate processes such as protein and DNA synthesis, which are sensitive to high pressure. Piezophilic bacteria have a high proportion of fatty acids in their cytoplasmic membrane, which allows membranes to remain functional and keep from gelling at high pressures. Although the study of nutrient acquisition  and metabolism within the piezosphere is still in its infancy, it is understood that most of the organic matter present are refractory complex polymers from the eutrophic zone. Both heterotrophic metabolism and autotrophic fixation are present within the piezosphere and additional research suggests significant metabolism of iron-bearing minerals and carbon monoxide. Additional research is required to fully understand and characterize piezosphere metabolism. Effect of high pressure on biological systems. High pressure has several effects on biological systems. The application of pressure results in equilibrium shifting towards state occupying small volume and it changes intermolecular distances and affects conformations. This  also has an effect on the functionality of the cells. Effect on nucleic acids. High pressure stabilizes hydrogen bonds and stacking interactions of the DNA. Thus it favours the double stranded duplex structure of the DNA. However, to carry out several processes like DNA replication, transcription and translation, the transition to single-strand structure is necessary which becomes difficult as high pressure increases the melting temperature, Tm. Thus, these processes may face difficulties. Effect on cell membranes. When pressure increases, the fluidity of the cell membrane is decreased as due to restrictions in volume they change their conformation and packing . This decreases the permeability of the cell membrane to water and different molecules. Effect on proteins. The macromolecules bearing the largest effect of pressure are proteins. Just like lipids, they change their conformation and packing to accommodate changes in pressure. This affects their multimeric conformation, stability and also the structure of their catalytic sites, which changes their functionality. In pressure-intolerant species, proteins tend to compact and unfold under high pressures as overall volume is reduced. Piezophilic proteins, however, tend to have less void space and smaller void spaces overall to mitigate compaction and unfolding pressures. Effect on enzymes. Due  to the functional nature of enzymes, piezophiles must maintain their activity to survive. High pressures tend to favor enzymes with higher flexibility at the cost of lower stability. Additionally, piezophilic enzymes often have high absolute (distinct from temperature or pressure) and relative catalytic activity. This allows the enzymes to maintain sufficient activity even with decreases due to temperature or pressure effects. Furthermore, some piezophilic enzymes have increasing catalytic activity with increasing pressures, though this is not a generalization for all piezophilic enzymes. Overall effect on cells. As a result of high pressure, several functions may be lost in organisms that  are pressure-intolerant. Effects can include loss of flagellar motility, enzyme function, and thus metabolism. It can also lead to cell death due to modifications in the cellular structure. High pressures also can cause an imbalance in oxidation and reduction reactions generating relatively high concentrations of reactive oxygen species (ROS). An increased amount of anti-oxidation genes and proteins are found in piezophiles to combat the ROS as they often cause cellular damage. Piezophiles: Optimum growth at high pressures. Though the high hydrostatic pressure has deleterious effects on organisms growing at atmospheric pressure, organisms which are solely found at high  pressure habitats at deep sea in fact need high pressures for their optimum growth. These are the piezophiles. Often their growth is inhibited at much higher pressures (such as 100MPa) compared to those organisms which normally grow at low pressures. The first obligate piezophile found was a psychrophilic bacteria called "Colwellia marinimaniae" strain M-41. It was isolated from a decaying amphipod "Hirondellea gigas" from the bottom of Mariana Trench. The first thermophilic piezophilic archaea "Pyrococcus yayanosii" strain CH1 was isolated from the Ashadze site, a deep sea hydrothermal vent. Strain MT-41 has a optimal growth pressure at 70MPa at 2  °C and strain CH1 has a optimal growth pressure at 52MPa at 98 °C. They are unable to grow at pressures lower than or equals to 20MPa, and both can grow at pressures above 100MPa.