A piezophile (from Greek "piezo-" for pressure and "-phile" for loving) is an

organism An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...

with optimal growth under high

hydrostatic pressure Hydrostatics is the branch of fluid mechanics that studies fluids at hydrostatic equilibrium and "the pressure in a fluid or exerted by a fluid on an immersed body". The word "hydrostatics" is sometimes used to refer specifically to water and o ...

, i.e., an organism that has its maximum rate of growth at a hydrostatic pressure equal to or above , when tested over all permissible

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

s. Originally, the term barophile was used for these organisms, but since the prefix "baro-" stands for

weight In science and engineering, the weight of an object is a quantity associated with the gravitational force exerted on the object by other objects in its environment, although there is some variation and debate as to the exact definition. Some sta ...

, the term piezophile was given preference. Like all definitions of

extremophile An extremophile () is an organism that is able to live (or in some cases thrive) in extreme environments, i.e., environments with conditions approaching or stretching the limits of what known life can adapt to, such as extreme temperature, press ...

s, the definition of piezophiles is

anthropocentric Anthropocentrism ( ) is the belief that human beings are the central or most important entity on the planet. The term can be used interchangeably with humanocentrism, and some refer to the concept as human supremacy or human exceptionalism. From a ...

, and humans consider that moderate values for hydrostatic pressure are those around 1 atm (= 0.1

MPa MPA or mPa may refer to: Academia Academic degrees * Master of Performing Arts * Master of Professional Accountancy * Master of Public Administration * Master of Public Affairs Schools * Mesa Preparatory Academy * Morgan Park Academy * M ...

= 14.7

psi Psi, PSI or Ψ may refer to: Alphabetic letters * Psi (Greek) (Ψ or ψ), the twenty-third letter of the Greek alphabet * Psi (Cyrillic), letter of the early Cyrillic alphabet, adopted from Greek Arts and entertainment * "Psi" as an abbreviat ...

), whereas those "extreme" pressures are the normal living conditions for those organisms. Hyperpiezophiles are organisms that have their maximum growth rate above 50 MPa (= 493 atm = 7,252 psi). Though the high hydrostatic pressure has deleterious effects on organisms growing at atmospheric pressure, these organisms which are solely found at high pressure habitats at deep sea in fact need high pressures for their optimum growth. Often their growth is able to continue 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 Mariana Trench is an oceanic trench located in the western Pacific Ocean, about east of the Mariana Islands; it is the deep sea, deepest oceanic trench on Earth. It is crescent-shaped and measures about in length and in width. The maxi ...

. The first thermophilic piezophilic archaea ''Pyrococcus yayanosii'' strain CH1 was isolated from the Ashadze site, a deep sea

hydrothermal vent Hydrothermal vents are fissures on the seabed from which geothermally heated water discharges. They are commonly found near volcanically active places, areas where tectonic plates are moving apart at mid-ocean ridges, ocean basins, and hot ...

. Strain MT-41 has an 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 equal to 20MPa, and both can grow at pressures above 100MPa.The current record for highest hydrostatic pressure where growth was observed is 140MPa shown by ''Colwellia marinimaniae'' MTCD1. The term "obligate piezophile" 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 The biosphere (), also called the ecosphere (), is the worldwide sum of all ecosystems. It can also be termed the zone of life on the Earth. The biosphere (which is technically a spherical shell) is virtually a closed system with regard to mat ...

(in terms of volume) is subject to high hydrostatic pressure, and the piezosphere comprises the

deep sea The deep sea is broadly defined as the ocean depth where light begins to fade, at an approximate depth of or the point of transition from continental shelves to continental slopes. Conditions within the deep sea are a combination of low tempe ...

(at the depth of 1,000 m and greater) plus the deep

subsurface In geology, bedrock is solid rock that lies under loose material (regolith) within the crust of Earth or another terrestrial planet. Definition Bedrock is the solid rock that underlies looser surface material. An exposed portion of bedro ...

(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, deep subsurface and hydrothermal vents in the seafloor are dominated by thermopiezophiles that prosper in temperatures above 45 °C (113 °F). 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 In materials science, a refractory (or refractory material) is a material that is resistant to decomposition by heat or chemical attack and that retains its strength and rigidity at high temperatures. They are inorganic, non-metallic compound ...

complex polymers from the eutrophic zone. Both

heterotroph A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...

ic metabolism and

autotroph An autotroph is an organism that can convert Abiotic component, abiotic sources of energy into energy stored in organic compounds, which can be used by Heterotroph, other organisms. Autotrophs produce complex organic compounds (such as carbohy ...

ic 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.

Piezophilic adaptations

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. Piezophiles employ several mechanisms to adapt themselves to these high hydrostatic pressures. They regulate gene expression according to pressure and also adapt their biomolecules to differences in pressure.

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.

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. In response to flucatuation in environment, they change their membrane structures. Piezophilic bacteria do so by varying their

acyl chain In chemistry, an acyl group is a moiety derived by the removal of one or more hydroxyl groups from an oxoacid, including inorganic acids. It contains a double-bonded oxygen atom and an organyl group () or hydrogen in the case of formyl group (). ...

length, by accumulating

unsaturated fatty acids In chemistry, in particular in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, f ...

, accumulating specific polar headgroups and branched fatty acids. Piezophilic archaea synthesize

archaeol Archaeol is a diether composed of two phytanyl chains linked to the sn-2 and sn-3 positions of glycerol. As its phosphate ester, it is a common component of the membranes of archaea. Structure and contrast with other lipids The 2,3-sn-glycerol ...

and cadarchaeol-based polar lipids, bipolar tetraether lipids, incorporate

cyclopentane Cyclopentane (also called C pentane) is a highly flammable alicyclic compound, alicyclic hydrocarbon with chemical formula C5H10, C5H10 and CAS number 287-92-3, consisting of a ring of five carbon atoms each bonded with two hydrogen atoms above and ...

rings and increase in unsaturation.

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. There are also changes in the various interactions between amino acids. In general, they are very resistant to pressure.

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 In chemistry and biology, reactive oxygen species (ROS) are highly Reactivity (chemistry), reactive chemicals formed from diatomic oxygen (), water, and hydrogen peroxide. Some prominent ROS are hydroperoxide (H2O2), superoxide (O2−), hydroxyl ...

(ROS). An increased amount of anti-oxidation genes and proteins are found in piezophiles to combat the ROS as they often cause cellular damage.

References

* {{Extremophile Aquatic ecology Bacteria