in the mineral olivine are exposed to seawater and react with it to form serpentine minerals and magnetite. non-equilibrium conditions are also associated with hydrothermal vents, volcanism, and geothermal activity. other processes that might provide habitats for life include roll front development in ore bodies, subduction, methane clathrate formation and decomposition, permafrost thawing, infrared radiation and seismic activity. humans also create new habitats for life, particularly through remediation of contaminants in the subsurface. energy sources life requires enough energy to construct adenosine triphosphate (atp). where there is sunlight, the main processes for capturing energy are photosynthesis (which harnesses the energy in sunlight by converting carbon dioxide into organic molecules) and respiration (which consumes those molecules and releases carbon dioxide). below the surface, the main source of energy is from chemical redox (reduction-oxidation) reactions. this requires electron donors (compounds that can be oxidized) and electron acceptors (compounds that can be reduced). an example of such a reaction is methane oxidation: ch4 + o2 → co2 + 2 h2o here ch4 is the donor and o2 is the acceptor. donors can be considered "edibles" and acceptors "breathables". the amount of energy that is released in a metabolic reaction depends on the redox potential of the chemicals involved. electron donors have negative potentials. from highest to lowest redox potential, some common donors available in the subsurface are organic matter, hydrogen, methane, reduced sulfur compounds, reduced iron compounds and ammonium. from most negative to least, some acceptors are oxygen, nitrates and nitrites, manganese and iron oxides, oxidized sulfur compounds, and carbon dioxide. of electron donors, organic matter has the most negative redox potential. it can consist of deposits from regions where sunlight is available or produced by local organisms. fresh material is more easily utilized than aged. terrestrial organic matter (mainly from plants) is typically harder to process than marine (phytoplankton). some organisms break down organic compounds using fermentation and hydrolysis, making it possible for others to convert it back to carbon dioxide. hydrogen is a good energy source, but competition tends to make it scarce. it is particularly rich in hydrothermal fluids where it is produced by serpentinization. multiple species can combine fermentation with methanogenesis and iron oxidation with hydrogen consumption. methane is mostly found in marine sediments, in gaseous form (dissolved or free) or in methane hydrates. about 20% comes from abiotic sources (breakdown of organic matter or serpentinization) and 80% from biotic sources (which reduce organic compounds such as carbon dioxide, carbon monoxide and acetate). over 90% of methane is oxidized by microbes before it reaches the surface; this activity is "one of the most important controls on greenhouse gas emissions and climate on earth." reduced sulfur compounds such as elemental sulfur, hydrogen sulfide (h2s) and pyrite (fes2) are found in hydrothermal vents in basaltic crust, where they precipitate out when metal-rich fluids contact seawater. reduced iron compounds in sediments are mainly deposited or produced by anaerobic reduction of iron oxides. the electron acceptor with the highest redox potential is oxygen. produced by photosynthesis, it is transported to the ocean floor. there, it is quickly taken up if there is a lot of organic material, and may only be present in the top few centimeters. in organic-poor sediments it can be found at greater depths, even to the oceanic crust. nitrate can be produced by degradation of organic matter or nitrogen fixation. oxygen and nitrate are derived from photosynthesis, so underground communities that utilize them are not truly independent of the surface. nutrients all life requires carbon, nitrogen, phosphorus and some trace elements such as nickel, molybdenum and vanadium. over 99.9% of earth's carbon is stored in the crust and its overlying sediments, but the availability of this carbon can depend on the oxidation state of the environment. organic carbon, nitrogen and phosphorus are primarily found in terrestrial sediments, which accumulate mainly in continental margins. organic carbon is mainly produced at the surface of the oceans with photosynthesis or washed into oceans with terrestrial sediments. only a small fraction is produced in the deep seas with chemosynthesis. when organic carbon sinks from the surface of the ocean to the seafloor, most of the organic carbon is consumed by organisms in seawater. only a small fraction of this sinking organic carbon can reach the seafloor and be available to the deep biosphere. deeper in the marine sediments, the organic content drops further. phosphorus is taken up by iron oxyhydroxides when basalts and sulfide rocks are weathered, limiting its availability. the availability of nutrients are limiting the deep biosphere, determining where and what type of deep organisms can thrive. pressure the push50 device keeps deep-sea samples at high pressure. atmospheric pressure is 101 kilopascals (kpa). in the ocean, the pressure increases at a rate of 10.5 kpa per m of depth, so at a typical depth of the sea floor (3800 m) the pressure is 38 megapascals (mpa). at these depths, the boiling point of water is over 400 °c. at the bottom of the mariana trench, the pressure is 110 mpa. in the lithosphere, the pressure increases by 22.6 kpa/m. the deep biosphere withstands pressures much higher than the pressure at the surface of the earth. an increased pressure compresses lipids, making membranes less fluid. in most chemical reactions, the products occupy more volume than the reactants, so the reactions are inhibited by pressure. nevertheless, some studies claim that cells from the surface are still active at a pressure of 1 gigapascal (gpa), about 10,000 times the standard atmospheric pressure. there are also piezophiles for which optimal growth occurs at pressures over 100 mpa, and some do not grow in pressures less than 50 mpa. as of 2019, most sampling of organisms from the deep ocean and subsurface undergo decompression when they are removed to the surface. this can harm the cells in a variety of ways, and experiments at surface pressures produce an inaccurate picture of microbial activity in the deep biosphere. a pressurized underwater sampler handler (push50) has been developed to maintain in situ pressure during sampling and afterwards in the laboratory. temperature high temperatures stress organisms, increasing the rates of processes that damage important molecules such as dna and amino acids. it also increases the energy requirements for repairing these molecules. however, cells can respond by changing the structure of these molecules to stabilize them. microbes can survive at temperatures above 100 °c if the pressure is high enough to keep the water from boiling. the highest temperature at which an organism has been cultured in a laboratory is 122 °c, under pressures of 20 mpa and 40 mpa. theoretical estimates for the highest temperature that can sustain life are around 150 °c. the 120 °c isotherm can be less than 10 m deep at mid-ocean ridges and seamounts, but in other environments such as deep-sea trenches it can be kilometers deep. about 39% by volume of ocean sediments is at temperatures between 40 °c and 120 °c. thermochronology data of precambrian cratons suggest that habitable temperature conditions of the subsurface in these settings range back to about a billion years maximum. the record-setting thermophile, methanopyrus kandlerii, was isolated from a hydrothermal vent. hydrothermal vents provide abundant energy and nutrients. several groups of archaea and bacteria thrive in the shallow seafloor at temperatures between 80 °c and 105 °c. as the environment becomes more energy-limited, such as being deeper, bacteria can survive but their number decreases. although microorganisms have been detected at temperatures up to 118 °c in cored sediments, attempts to isolate the organisms have failed. there can also be depth intervals with less cells than the deeper part of the location. reasons for such 'low- or no-cell intervals' are still unknown but may be related to the underground flow of hot fluid. in deep oil reservoirs, no microbial activity has been seen hotter than 80 °c. living with energy limitation in most of the subsurface, organisms live in conditions of extreme energy and nutrient limitation. this is far from the conditions in which cells are cultured in labs. a lab culture goes through a series of predictable phases. after a short lag phase, there is a period of exponential growth in which the population can double in as little as 20 minutes. a death phase follows in which almost all the cells die off. the remainder enter an extended stationary phase in which they can last for years without further input of substrate. however, each live cell has 100 to 1000 dead cells to feed on, so they still have abundant nutrients compared to the subsurface. in the subsurface, cells catabolize (break down molecules for energy or building materials) 10,000 to one million times slower than at the surface. biomass may take centuries or millennia to turn over. there is no known limit to the age that cells could reach. the viruses that are present could kill cells and there may be grazing by eukaryotes, but there is no evidence of that. it is difficult to establish clear limits on the energy needed to keep cells alive but not growing. they need energy to perform certain basic functions like the maintenance of osmotic pressure and maintenance of macromolecules such as enzymes and rna (e.g., proofreading and synthesis). however, laboratory estimates of the energy needed are several orders of magnitude greater than the energy supply that appears to sustain life underground. it was thought, at first, that most underground cells are dormant. however, some extra energy is required to come out of dormancy. this is not a good strategy in an environment where the energy sources are stable over millions of years but decreasing slowly. the available evidence suggests that most cells in the subsurface are active and viable. a low-energy environment favors cells with minimal self-regulation, because there are no changes in the environment that they need to respond to. there could be low-energy specialists. however, there is unlikely to be strong evolutionary pressure for such organisms to evolve because of the low turnover and because the environment is a dead end. diversity the biomass in the deep subsurface is about 15% of the total for the biosphere. life from all three domains (archaea, bacteria, and eukarya) have been found in the deep subsurface; indeed, the deep subsurface accounts for about 90% of all the biomass in archaea and bacteria. the genetic diversity is at least as great as that on the surface. in the ocean, plankton species are distributed globally and are constantly being deposited almost everywhere. quite different communities are found even in the top of ocean floor, and species diversity decreases with depth. however, there are still some taxa that are widespread in the subsurface. in marine sediments, the main bacterial phyla are "candidatus atribacteria" (formerly op9 and js1), pseudomonadota, chloroflexota, and planctomycetota. members of archaea were first identified using metagenomic analysis; some of them have since been cultured and they have acquired new names. the deep sea archaeal group (dsag) became the marine benthic group b (mbg-b) and is now a proposed phylum "lokiarchaeota". along with the former ancient archaeal group (aag) and marine hydrothermal vent group (mhvg), "lokiarchaeota" is part of a candidate superphylum, asgard. other phyla are "bathyarchaeota" (formerly the miscellaneous chrenarchaeotal group), nitrososphaerota (formerly thaumarchaeota or marine group i), and euryarchaeota (including "hadesarchaea", archaeoglobales and thermococcales). a related clade, anaerobic methanotrophic archaea (anme), is also represented. other bacterial phyla include thermotogota. in the continental subsurface, the main bacterial groups are pseudomonadota and bacillota while the archaea are mainly methanomicrobia and nitrososphaerota. other phyla include "bathyarchaeota" and "aigarchaeota", while bacterial phyla include aquificota and nitrospirota. the eukarya in the deep biosphere include some multicellular life. in 2009 a species of nematode, halicephalobus mephisto, was discovered in rock fissures more than a kilometer down a south african gold mine. nicknamed the "devil worm", it may have been forced down along with pore water by earthquakes. other multicellular organisms have since been found, including fungi, platyhelminthes (flatworms), rotifera, annelida (ringed worms) and arthropoda. however, their range may be limited because sterols, needed to construct membranes in eukarya, are not easily made in anaerobic conditions. viruses are also present in large numbers and infect a diverse range of microbes in the deep biosphere. they may contribute significantly to cell turnover and transfer