The subtle body in Indian mysticism, from a yoga manuscript in Braj Bhasa language, 1899. A row of chakras is depicted from the base of the spine up to the crown of the head.
A subtle body is a "quasi material" aspect of the human body, being neither solely physical nor solely spiritual, according to various esoteric, occult, and mystical teachings. This contrasts with the mind–body dualism that has dominated Western thought. The subtle body is important in the Taoism of China and Dharmic religions such as Hinduism, Buddhism, and Jainism, mainly in the branches which focus on tantra and yoga, where it is known as the Sūkṣma-śarīra (Sanskrit: सूक्ष्म शरीर).
However, while mostly associated with Asian cultures, non-dualistic
approaches to the mind and body are found in many parts of the world.
Subtle body concepts and practices can be identified as early as 2nd century BCE in Taoist texts found in the Mawangdui tombs. It was "evidently present" in Indian thought as early as the 4th to 1st century BCE when the Taittiriya Upanishad described the Panchakoshas, a series of five interpenetrating sheaths of the body. A fully formed subtle body theory did not develop in India until the tantric movement that affected all its religions in the Middle Ages. In Indo-Tibetan Buddhism,
the correlation of the subtle body to the physical body is viewed
differently according to school, lineage and scholar, but for completion stage in yoga, it is visualised within the body. The subtle body consists of focal points, often called chakras, connected by channels, often called nadis, that convey subtle breath, often called prana. Through breathing and other exercises, a practitioner may direct the subtle breath to achieve supernormal powers, immortality, or liberation.
Subtle body in the Western tradition is called the body of light. The concept derives from the philosophy of Plato: the word 'astral' means 'of the stars'; thus the astral plane consists of the Seven Heavens of the classical planets. Neoplatonists Porphyry and Proclus elaborated on Plato's description of the starry nature of the human psyche. Throughout the Renaissance, philosophers and alchemists, healers including Paracelsus and his students, and natural scientists such as John Dee,
continued to discuss the nature of the astral world intermediate
between earth and the divine. The concept of the astral body or body of
light was adopted by 19th and 20th-century ceremonial magicians.
The Theosophy
movement was the first to translate the Sanskrit term as 'subtle body',
although their use of the term is quite different from Indic usage as
they synthesize Western and Eastern traditions. This makes the term
problematic for modern scholars, especially as the Theosophist view
often influences New Age and holistic medicine perspectives. Western scientists have started to explore the subtle body concept in research on meditation.
Asian religions
The Yogic, Tantric and other systems of Hinduism, Vajrayana Buddhism, as well as Chinese Taoist alchemy contain theories of subtle physiology with focal points (chakras, acupuncture points) connected by a series of channels (nadis, meridians) that convey subtle breath (prana, vayu, ch'i, ki, lung).
These invisible channels and points are understood to determine the
characteristics of the visible physical form. By understanding and
mastering the subtlest levels of reality one gains mastery over the
physical realm. Through breathing and other exercises, the practitioner
aims to manipulate and direct the flow of subtle breath, to achieve
supernormal powers (siddhis) and attain higher states of consciousness, immortality, or liberation.
Hinduism
An illustration of a subtle body system of seven chakras connected by three major nadi channels, as commonly adopted by contemporary yoga
Early concepts of the subtle body (Sanskrit: sūkṣma śarīra) appeared in the Upanishads, including the Brhadaranyaka Upanishad and the Katha Upanishad. The Taittiriya Upanishad describes the theory of five koshas or sheaths, though these are not to be thought of as concentric layers, but interpenetrating at successive levels of subtlety:
The anna-maya ("food body", physical body, the grossest level),
The prana-maya (body made of vital breath or prana),
Subtle internal anatomy included a central channel (nadi). Later Vedic texts called samhitas and brahmanas contain a theory of five "winds" or "breaths" (vayus, pranas):
Prāṇa, associated with inhalation
Uḍāna, associated with exhalation
Vyāna, associated with distribution of breath within the body
Samāna, associated with digestion
Apāna, associated with excretion of waste
Later
A millennium later, these concepts were adapted and refined by various spiritual traditions. The similar concept of the Liṅga Śarīra is seen as the vehicle of consciousness in later Samkhya, Vedanta, and Yoga, and is propelled by past-life tendencies, or bhavas. Linga can be translated as "characteristic mark" or "impermanence" and the Vedanta term sarira as "form" or "mould". Karana or "instrument" is a synonymous term. In the Classical Samkhya system of Isvarakrsna (ca. 4th century CE), the Lińga is the characteristic mark of the transmigrating entity. It consists of twenty-five tattvas from eternal consciousness down to the five organs of sense, five of activity (buddindriya or jñānendriya, and karmendriya respectively) and the five subtle elements that are the objects of sense (tanmatras) The Samkhyakarika says:
The subtle body (linga), previously arisen, unconfined, constant, inclusive of the great one (mahat) etc, through the subtle elements, not having enjoyment, transmigrates, (because of) being endowed with bhavas
("conditions" or "dispositions").
As a picture (does) not (exist) without a support, or as a shadow (does)
not (exist) without a post and so forth; so too the instrument (linga or karana) does not exist without that which is specific (i.e., a subtle body).
— Samkhyakarika, 60–81
The classical Vedanta tradition developed the theory of the five bodies into the theory of the koshas "sheaths" or "coverings" which surround and obscure the self (atman). In classical Vedanta these are seen as obstacles to realization and traditions like Shankara's Advaita Vedanta had little interest in working with the subtle body.
Tantra
In Tantra traditions meanwhile (ShaivaKaula, Kashmir Shaivism and Buddhist Vajrayana), the subtle body was seen in a more positive light, offering potential for yogic practices which could lead to liberation. Tantric traditions contain the most complex theories of the subtle body, with sophisticated descriptions of energy nadis (literally "stream or river", channels through which vayu and prana flows) and chakras, points of focus where nadis meet.
The main channels, shared by both Hindu and Buddhist systems, but
visualised entirely differently, are the central (in Hindu systems: sushumna; in Buddhist: avadhuti), left and right (in Hindu systems: ida and pingala; Buddhist: lalana and rasana). Further subsidiary channels are said to radiate outwards from the chakras, where the main channels meet.
Chakra systems vary with the tantra; the Netra Tantra describes six chakras, the Kaulajñana-nirnaya describes eight, and the Kubjikamata Tantra describes seven (the most widely known set).
In the Dzogchen tradition of Tibetan Buddhism,
the subtle body takes a different form. More specifically, the
tradition points to four areas of particularly concentration of bodily energy – viz. the heart (tsitta), where the enlightened energy resides; the "luminous channels" (‘od rtsa), through which the energy flows; the skull (dung khang), where it spreads before finally being released through the fourth hot-spot, namely the eyes (tsakshu / briguta). Flavio Geisshuesler, who has studied the functioning of the Dzogchen subtle body in the context of the practice of sky-gazing,
argues that many of the specific motifs that appear in the tradition's
conception of the body are of pre-Buddhist origin. More specifically, he
notes that the Dzogchen body's motifs of "deer-hearts, silk-channels,
buffalo-horns, or far-reaching lassos [...] reproduce the terminology of
the hunting of animalistic vitality as if internalizing the quest for
precious substances."
Modern
The modern Indian spiritual teacher Meher Baba
stated that the subtle body "is the vehicle of desires and vital
forces". He held that the subtle body is one of three bodies with which
the soul must cease to identify with in order to realize God.
A Tibetan illustration of the subtle body showing the central channel and two side channels connecting five chakras
In Buddhist Tantra, the subtle body is termed the "innate body" (nija-deha) or the "uncommon means body" (asadhdrana-upayadeha), or sūkṣma śarīra, rendered in Tibetan as traway-lu (transliterated phra ba’i lus). The subtle body is sometimes known as manomaya-kāya, the “body made of mind” and is the means for synchronising the body and the mind, particularly during meditation.
The subtle body consists of thousands of subtle energy channels (nadis), which are conduits for energies or "winds" (lung or prana) and converge at chakras. According to Dagsay Tulku Rinpoche, there are three main channels (nadis),
central, left and right, which run from the point between the eyebrows
up to the crown chakra, and down through all seven chakras to a point
two inches below the navel.
Lati Rinbochay describes the subtle body as consisting of 72,000
channels, various winds and a white and a red drop whilst a further very
subtle body is a wind abiding in a drop at the centre of the heart
chakra. The central channel is then described as being squeezed by two
channels that encircle it at each chakra and thrice at the heart chakra,
ensuring the winds do not move upward or downward until death.
Buddhist tantras generally describe four or five chakras in the shape of a lotus with varying petals. For example, the Hevajra Tantra (8th century) states:
In
the Center [i.e. chakra] of Creation [at the sexual organ] a sixty-four
petal lotus. In the Center of Essential Nature [at the heart] an eight
petal lotus. In the Center of Enjoyment [at the throat] a sixteen petal
lotus. In the Center of Great Bliss [at the top of the head] a
thirty-two petal lotus.
In contrast, the historically later Kalachakra tantra describes six chakras.
Other spiritual traditions teach about a mystical or divine body, such as "the most sacred body" (wujud al-aqdas) and "true and genuine body" (jism asli haqiqi) in Sufism, the meridian system in Chinese religion, and "the immortal body" (soma athanaton) in Hermeticism.
The body of light is elaborated on according to various Western esoteric, occult, and mystical teachings. Other terms used for this body include body of glory, spirit-body, radiant body, luciform body, augoeides ('radiant'), astroeides ('starry' or 'sidereal body'), and celestial body.
The concept derives from the philosophy of Plato: the word 'astral' means 'of the stars'; thus the astral plane consists of the Seven Heavens of the classical planets. The idea is rooted in common worldwide religious accounts of the afterlife in which the soul's
journey or "ascent" is described in such terms as "an ecstatic,
mystical or out-of body experience, wherein the spiritual traveller
leaves the physical body and travels in their body of light into
'higher' realms."
In the 19th century, H. P. Blavatsky founded the esoteric religious system of Theosophy, which attempted to restate Hindu and Buddhist philosophy for the Western world. She adopted the phrase "subtle body" as the English equivalent of the Vedantic sūkṣmaśarīra, which in Adi Shankara's
writings was one of three bodies (physical, subtle, and causal).
Geoffrey Samuel notes that theosophical use of these terms by Blavatsky
and later authors, especially C. W. Leadbeater, Annie Besant and Rudolf Steiner (who went on to found Anthroposophy), has made them "problematic"
to modern scholars, since the Theosophists adapted the terms as they
expanded their ideas based on "psychic and clairvoyant insights",
changing their meaning from what they had in their original context in
India.
Post-theosophists
The later theosophical arrangement was taken up by Alice Bailey, and from there found its way into the New Age worldview and the human aura. Other authors treated the subtle body in varying ways. Max Heindel divided the subtle body into the Vital Body made of Ether; the Desire body, related to the Astral plane; and the Mental body. Barbara Brennan's account of the subtle bodies in her books Hands of Light and Light Emerging refers to the subtle bodies as "layers" in the "Human Energy Field" or aura.
Fourth Way
Subtle bodies are found in the "Fourth Way" teachings of Gurdjieff and Ouspensky,
who write that one can create a subtle body, and hence achieve
post-mortem immortality, through spiritual or yogic exercises. The
"soul" in these systems is not something one is born with, but developed
through esoteric practice to acquire complete understanding and to
perfect the self. According to the historian Bernice Rosenthal, "In
Gurdjieff's cosmology our nature is tripartite and is composed of the
physical (planetary), emotional (astral) and mental (spiritual) bodies;
in each person one of these three bodies ultimately achieves dominance."
The "divine body" represents a fourth way, and the ultimate task of the
teachings is to harmoniously develop the four ways into a single way.
Meditation research
Western
scientists have started to explore the subtle body concept in relation
to research on meditation. The subtle body model can be cross-referenced
onto modern maps of the central nervous system, and applied in research on meditation.
Environments in which subsurface life has been found
The deep biosphere is the part of the biosphere
that resides below the first few meters of the ocean's surface. It
extends 10 kilometers below the continental surface and 21 kilometers
below the sea surface, at temperatures that may reach beyond 120 °C
(248 °F) which is comparable to the maximum temperature where a metabolically active organism has been found. It includes all three domains of life and the genetic diversity rivals that on the surface.
The first indications of deep life came from studies of oil
fields in the 1920s, but it was not certain that the organisms were
indigenous until methods were developed in the 1980s to prevent
contamination from the surface. Samples are now collected in deep mines
and scientific drilling programs in the ocean and on land. Deep observatories have been established for more extended studies.
Near the surface, living organisms consume organic matter and
breathe oxygen. Lower down, these are not available, so they make use of
"edibles" (electron donors) such as hydrogen (released from rocks by various chemical processes), methane (CH4), reduced sulfur compounds, and ammonium (NH4). They "breathe" electron acceptors such as nitrates and nitrites, manganese and iron oxides, oxidized sulfur compounds and carbon dioxide (CO2).
There is very little energy at greater depths, so metabolisms are up to
a million times slower than at the surface. Cells may live for
thousands of years before dividing and there is no known limit to their
age.
The subsurface accounts for about 90% of the biomass across two domains of life, Archaea and Bacteria, and 15% of the total for the biosphere. Eukarya are also found, including some multicellular life - fungi and animals (nematodes, flatworms, rotifers, annelids, and arthropods). Viruses are also present and infect the microbes.
Definition
The deep biosphere is an ecosystem of organisms and their living space in the deep subsurface. For the seafloor, an operational definition of deep subsurface is the region that is not bioturbated by animals; this is generally about a meter or more below the surface. On continents, it is below a few meters, not including soils. The organisms in this zone are sometimes referred to as intraterrestrials.
A subset of the deep biosphere found at depths where pressure and heat
greatly exceed that survivable by surface life was delineated and named
by Thomas Gold in a 1992 paper titled, "The Deep, Hot Biosphere."
Early discoveries and ideas
At the University of Chicago
in the 1920s, geologist Edson Bastin enlisted the help of
microbiologist Frank Greer in an effort to explain why water extracted
from oil fields contained hydrogen sulfide and bicarbonates.
These chemicals are normally created by bacteria, but the water came
from a depth where the heat and pressure were considered too great to
support life. They were able to culture anaerobic sulfate-reducing bacteria from the water, demonstrating that the chemicals had a bacterial origin.
Also in the 1920s, Charles Lipman, a microbiologist at the University of California, Berkeley, noticed that bacteria that had been sealed in bottles for 40 years could be reanimated – a phenomenon now known as anhydrobiosis.
He wondered whether the same was true of bacteria in coal seams. He
sterilized samples of coal, wetted them, crushed them and then succeeded
in culturing bacteria from the coal dust. One sterilization procedure,
baking the coal at 160 °C (320 °F) for up to 50 hours, actually
encouraged their growth. He published the results in 1931.
The first studies of subsurface life were conducted by Claude E. Zobell, the "father of marine microbiology", in the late 1930s to the 1950s. Although the coring depth was limited, microbes were found wherever the sediments were sampled. With increasing depth, aerobes gave way to anaerobes.
Most biologists dismissed the subsurface microbes as contamination, especially after the submersible Alvin sank in 1968 and the scientists escaped, leaving their lunches behind. When Alvin was recovered, the lunches showed no sign of microbial decay.
This reinforced a view of the deep sea (and by extension the
subsurface) as a lifeless desert. The study of the deep biosphere, like
many bacteria, was dormant for decades; an exception is a group of
Soviet microbiologists who began to refer to themselves as geomicrobiologists.
Interest in subsurface life was renewed when the United States Department of Energy was looking for a safe way of burying nuclear waste, and Frank J. Wobber
realized that microbes below the surface could either help by degrading
the buried waste or hinder by breaching the sealed containers. He
formed the Subsurface Science Program to study deep life. To address the
problem of contamination, special equipment was designed to minimize
contact between a core sample and the drilling fluid that lubricates the drill bit. In addition, tracers were added to the fluid to indicate whether it penetrated the core. In 1987, several boreholes were drilled near the Savannah River Site, and microorganisms were found to be plentiful and diverse at least 500 metres below the surface.
From 1983 until now, microbiologists have analyzed cell abundances in drill cores from the International Ocean Discovery Program (originally the Ocean Drilling Program). A group led by John Parkes of the University of Bristol reported concentrations of 104 to 108 cells per gram of sediment down to depths of 500 metres (agricultural soils contain about 109 cells per gram). This was initially met with skepticism, and it took them four years to publish their results.
In 1992, Thomas Gold
published a paper titled "The Deep, Hot Biosphere" suggesting that
microbial life was widespread throughout the subsurface, existing in
pore spaces between grains of rocks. He also published a book similarly titled The Deep Hot Biosphere.
According to one paper, he "pioneered" the idea the hydrocarbons could
sustain life to "known depths of 10km and possibly down to 300km", if
the temperature was not over a hypothetical maximum of 150°C.
Gold also suggested, largely incorrectly, that the deep biosphere is
sustained by hydrocarbons geologically produced by the subsurface, or
their derivatives. According to the paper, Gold's proposals helped to inspire later generations of scientists.
In 1998, William Whitman and colleagues published a summary of twelve years of data in the Proceedings of the National Academy of Sciences. They estimated that up to 95% of all prokaryotes (archaea and bacteria) live in the deep subsurface, with 55% in the marine subsurface and 39% in the terrestrial subsurface.
In 2002, Ocean Drilling Program Leg 201 was the first to be motivated
by a search for deep life. Most of the previous exploration was on
continental margins, so the goal was to drill in the open ocean for
comparison. In 2016, International Ocean Discovery Program Leg 370 drilled into the marine sediment of the NankaiAccretionary Prism and observed 102 vegetative cells per cm3 at 118 °C.
Scientific methods
The
present understanding of subsurface biology was made possible by
numerous advances in technology for sample collection, field analysis,
molecular science, cultivation, imaging and computation.
Sample collection
Schematic of an expedition aboard the Japanese drilling ship D/V ChikyūResearcher sampling fluid from a deep mine
Microbes from the ocean floor can sampled by drilling boreholes and collecting cores. The methods must be adapted to different types of rock, and the cost of drilling limits the number of holes that can be drilled. Microbiologists have made use of scientific drilling programs: the Ocean Drilling Program (ODP), which used the JOIDES Resolution drilling platform, and the Integrated Ocean Drilling Program (IODP), which used the Japanese ship Chikyū.
To allow continuous underground sampling, various kinds of
observatories have been developed. On the ocean floor, the Circulation
Obviation Retrofit Kit (CORK) seals a borehole to cut off the influx of
seawater. An advanced version of CORK is able to seal off multiple sections of a drill hole using "packers", rubber tubes that inflate to seal the space between the drill string and the wall of the borehole.
In sediments, the Simple Cabled Instrument for Measuring Parameters In-Situ (SCIMPI) is designed to remain and take measurements after a borehole has collapsed. Packers are also used in the continental subsurface, along with devices such as the flow-through in situ reactor (FTISR). Various methods are used to extract fluids from these sites, including passive and osmotic gas samplers and U-tube systems. In narrow (less than 50 mm) holes, polyamide tubes with a back-pressure valve can be lowered to sample an entire column of fluid.
Field analysis and manipulation
Some methods analyze microbes in situ rather than extracting them from the subsurface. In biogeophysics,
the effects of microbes on properties of geological materials are
remotely probed using electrical signals. Microbes can be tagged using a
stable isotope, such as carbon-13, and then re-injected in the ground to see where they go. A "push-pull" method involves injection of a fluid into an aquifer
and extraction of a mixture of injected fluid with the ground water;
the latter can then be analyzed to determine what chemical reactions
occurred.
Deep
microorganisms change the chemistry of their surroundings through the
nutrients they consume and the wastes they produce from metabolic activity.
Therefore scientists can estimate the activities of the deep
microorganisms by measuring the chemical compositions of subsurface
samples. Complementary techniques include measuring the isotope compositions of the chemicals or the related minerals.
Conditions for life
For life to have metabolic activity, it must be able to take advantage of a thermodynamic disequilibrium in the environment. This can occur when rocks from the mantle that are rich 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 convertingcarbon 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 + 2 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.
On average, atmospheric pressure at sea level is about 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 are at temperatures between 40 °C and 120 °C. Thermochronology data of Precambriancratons suggest that habitable temperature conditions of the subsurface in these settings range back to about a billion years maximum.[54]
The record-setting thermophile, Methanopyrus kandlerii, was isolated from a hydrothermal vent, which 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. Aerobic microbes are also present; methane-feeding bacteria will break down nitrites into nitrogen and oxygen, and then use the oxygen to split methane
for energy. Some of the oxygen produced this way will leak out of the
cells and into the surrounding environment, where it will benefit other
oxygen-dependent microorganisms.
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,
but some of them have since been cultured and 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 of genetic information
between cells.
Habitats
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.
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.
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 settle 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 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.
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 here, 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.[65]
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, which 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.
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 Nitrososphaerotaarchaea. 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 Chloroflexibacterial phylum 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.
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