of genetic information between cells. habitats environments in which subsurface life has been found. life has been found at depths of 5 km in continents and 10.5 km below the ocean surface. in 1992, thomas gold calculated that if the estimated pore space of the terrestrial land mass down to 5 km depth was filled with water, and if 1% of this volume were microbial biomass, it would be enough living matter to cover earth's land surface with a 1.5 m thick layer. the estimated volume of the deep biosphere is 2–2.3 billion cubic kilometers, about twice the volume of the oceans. ocean floor the main types of habitat below the seafloor are sediments and igneous rock. the latter may be partially altered and coexist with its alteration products such as sulfides and carbonates. in rock, chemicals are mainly carried through an aquifer system that cycles all of the ocean's water every 200,000 years. in sediments below the top few centimeters, chemicals mainly spread by the much slower process of diffusion. sediments nearly all of the seafloor is covered by marine sediments. they can vary in thickness from centimeters near ocean ridges to over 10 kilometers in deep trenches. in the mid-ocean, coccoliths and shells settling down from the surface form oozes, while near shore sediment is carried from the continents by rivers. minerals from hydrothermal vents and wind-blown particles also contribute. as organic matter is deposited and buried, the more easily utilized compounds are depleted by microbial oxidation, leaving the more recalcitrant compounds. thus, the energy available for life declines. in the top few meters, metabolic rates decline by 2 to 3 orders of magnitude, and throughout the sediment column cell numbers decline with depth. sediments form layers with different conditions for life. in the top 5–10 centimeters, animals burrow, reworking the sediment and extending the sediment-water interface. the water carries oxygen, fresh organic matter and dissolved metabolites, resulting in a heterogenous environment with abundant nutrients. below the burrowed layer is a layer dominated by sulfate reduction. below that, the anaerobic reduction of methane is facilitated by sulfate in the sulfate-methane transition zone (smtz). once the sulfates are depleted, methane formation takes over. the depth of the chemical zones depends on the rate that organic matter is deposited. where it is rapid, oxygen is taken up rapidly as organic matter is consumed; where slow, oxygen can persist much deeper because of the lack of nutrients to oxidize. ocean sediment habitats can be divided into subduction zones, abyssal plains, and passive margins. at a subduction zone, where one plate is diving under another, a thick wedge of sediment tends to form. at first the sediment has 50 to 60 percent porosity; as it is compressed, fluids are expelled to form cold seeps or gas hydrates. abyssal plains are the region between continental margins and mid-ocean ridges, usually at depths below 4 kilometers. the ocean surface is very poor in nutrients such as nitrate, phosphate and iron, limiting the growth of phytoplankton; this results in low sedimentation rates. the sediment tends to be very poor in nutrients, so not all the oxygen is consumed; oxygen has been found all the way down to the underlying rock. in such environments, cells are mostly either strictly aerobic or facultative anaerobic (using oxygen where available but able to switch to other electron acceptors in its absence) and they are heterotrophic (not primary producers). they include pseudomonadota, chloroflexota, marine group ii archaea and lithoautotrophs in the nitrososphaerota phylum. fungi are diverse, including members of the ascomycota and basidiomycota phyla as well as yeasts passive margins (continental shelves and slopes) are under relatively shallow water. upwelling brings nutrient-rich water to the surface, stimulating abundant growth of phytoplankton, which then settles to the bottom (a phenomenon known as the biological pump). thus, there is a lot of organic material in the sediments, and all the oxygen is used up in its consumption. they have very stable temperature and pressure profiles. the population of microbes is orders of magnitude greater than in the abyssal plains. it includes strict anaerobes including members of the chloroflexi phylum, "ca. atribacteria", sulfate-reducing bacteria, and fermenters, methanogens and methanotrophs in the archaea. fungi are less diverse than in abyssal plains, mainly including ascomycota and yeasts. viruses in the inoviridae, siphoviridae, and lipothrixviridae families have been identified. rocks schematic of the sumatra trench and the associated island arcs and back-arc regions ocean crust forms at mid-ocean ridges and is removed by subduction. the top half kilometer or so is a series of basaltic flows, and only this layer has enough porosity and permeability to allow fluid flow. less suitable for life are the layers of sheeted dikes and gabbros underneath. mid-ocean ridges are a hot, rapidly changing environment with a steep vertical temperature gradient, so life can only exist in the top few meters. high-temperature interactions between water and rock reduce sulfates, producing abundant sulfides that serve as energy sources; they also strip the rock of metals that can be sources of energy or toxic. along with degassing from magma, water interactions also produce a lot of methane and hydrogen. no drilling has yet been accomplished, so information on microbes comes from samples of hydrothermal fluids coming out of vents. about 5 kilometers off the ridge axis, when the crust is about 1 million years old, ridge flanks begin. characterized by hydrothermal circulation, they extend to about 80 million years in age. this circulation is driven by latent heat from the cooling of crust, which heats seawater and drives it up through more permeable rock. energy sources come from alteration of the rock, some of which is mediated by living organisms. in the younger crust, there is a lot of iron and sulfur cycling. sediment cover slows the cooling and reduces the flow of water. there is little evidence of microbe activity in older (more than 10 million year old) crust. near subduction zones, volcanoes can form in island arcs and back-arc regions. the subducting plate releases volatiles and solutes to these volcanoes, resulting in acidic fluids with higher concentrations of gases and metals than in the mid-ocean ridge. it also releases water that can mix with mantle material to form serpentinite. when hotspot volcanoes occur in the middle of oceanic plates, they create permeable and porous basalts with higher concentrations of gas than at mid-ocean ridges. hydrothermal fluids are cooler and have a lower sulfide content. iron-oxidizing bacteria create extensive deposits of iron oxides. porewater microorganisms live in the cracks, holes and empty space inside sediments and rocks. such empty space provides water and dissolved nutrients to the microorganisms. note that as the depth increases, there are less nutrients in the porewater as nutrients are continuously consumed by microorganisms. as the depth increases, the sediment is more compact and there is less space between mineral grains. as a result, there is less porewater per volume. the environment gets drier and drier when sediments are transitioned into rocks. at this stage, water can also be a limiting factor to the deep biosphere. continents continents have a complex history and a great variety of rocks, sediments and soils; the climate on the surface, temperature profiles and hydrology also vary. most of the information on subsurface life comes from a small number of sampling sites that are mainly in north america. with the exception of ice cores, densities of cells decline steeply with depth, decreasing by several orders of magnitude. in the top one or two meters of soils, organisms depend on oxygen and are heterotrophs, depending on the breakdown of organic carbon for their nutrition, and their decline in density parallels that of the organic material. below that, there is no correlation, although both cell density and organic content declines by a further five orders of magnitude or so (by contrast, there is a correlation in ocean sediments). increasing depth, temperature and salinity do correlate with declining cell numbers, although the rates depend strongly on type of crust and rate of groundwater recharge. microbes have been found in sedimentary rocks down to about 3 kilometers, the deepest sampled. there is a lot of diversity, although the deepest tend to be iron(iii)- or sulfate-reducing bacteria that use fermentation and can thrive in high temperature and salinity. even more salt-tolerant halophiles have been found in deep salt deposits, and such deposits are found all over the world. in 2019 microbial organisms were discovered living 2,400 meters below the surface, breathing sulfur and eating rocks such as pyrite as their regular food source. the discovery occurred in the oldest known water on earth. a study of biosignatures in vein mineral samples from more than 30 deep mines in the fennoscandian shield proves that signatures of ancient life are omnipresent across the shield. humans have accessed deep aquifers in igneous rocks for a variety of purposes including groundwater extraction, mining, and storage of hazardous wastes. most or all of these aquifers host microbes. at all the sites that have been tested, hydrogen, methane and carbon dioxide have been found. hydrogen-based communities of prokaryotes have also been found in hot springs and hydrothermal systems. a variety of mechanisms have been proposed for the production of hydrogen, some of which would be independent of photosynthesis. ecology the purplish rod-shaped cells, a few microns long, are "candidatus desulforudis audaxviator". one species of bacteria, "candidatus desulforudis audaxviator", is the first known to comprise a complete ecosystem by itself. it was found 2.8 kilometers below the surface in a gold mine near johannesburg, south africa. in alkaline water at a temperature of about 60 °c, with no access to oxygen, it gets energy by reducing sulfate, its nitrogen from ammonia molecules and ammonium ions, and its carbon from carbon dioxide or formate. stable isotope records of (secondary) fracture-lining minerals of the continental igneous rock-hosted deep biosphere point to long-term occurrence of methanogenesis, methanotrophy and sulfate reduction. morphological and spatiotemporal relations point to potential syntrophic relation of these prokaryotic metabolisms with fungi. other ecosystems have multiple interdependent species. they can be divided into autotrophs, which derive energy from non-living sources, and heterotrophs, which feed on autotrophs or their remains. some organisms engage in syntrophy, where one organism lives off the byproducts of another's metabolic activity. at the surface, most autotrophs use photosynthesis, but where there is no light, chemoautotrophs make use of chemical energy. in marine sediments where oxygen is available, a major group of chemoautotrophs is ammonia-oxidizing nitrososphaerota. it supports 19% of the heterotrophic production. in some environments such as abyssal pacific ocean sediments, the supply of ammonia dwindles with depth; but in other environments ammonia actually increases because heterotrophic bacteria, living on organic material, remineralize the ammonia. this interdependence of the heterotrophic bacteria and nitrososphaerota is an example of syntrophy. however, some nitrososphaerota are mixotrophic, able to use both organic matter and carbon dioxide for carbon. in anoxic sediments, hydrogen is an important "edible". members of the chloroflexi draw energy from it to produce acetate by reducing carbon dioxide or organic matter (a process known as acetogenesis). metal-reducing and sugar-fermenting bacteroidetes produce propionate, among other compounds, and this is fermented by "ca. atribacteria" to produce hydrogen. in upper sediments, sulfate-reducing bacteria take up most of the hydrogen, while in lower sediments the sulfate is depleted and methanogens dominate. in the sulfate-methane transition zone (smtz), anaerobic methanotrophic (anme) archaea form consortia with sulfate-reducing bacteria.