Life has traditionally been seen as driven by energy from the sun, but deep-sea organisms have no access to sunlight, so biological communities around hydrothermal vents must depend on nutrients found in the dusty chemical deposits and hydrothermal fluids in which they live. Previously,
benthic oceanographers assumed that vent organisms were dependent on
marine snow, as deep-sea organisms are. This would leave them dependent on plant life and thus the sun. Some hydrothermal vent organisms do consume this "rain", but with only such a system, life forms would be sparse. Compared to the surrounding sea floor, however, hydrothermal vent zones have a density of organisms 10,000 to 100,000 times greater. The hydrothermal vents are recognized as a type of
chemosynthetic based
ecosystems (CBE) where primary productivity is fuelled by chemical compounds as energy sources instead of light (
chemoautotrophy). Hydrothermal vent communities are able to sustain such vast amounts of life because vent organisms depend on chemosynthetic bacteria for food. The water from the hydrothermal vent is rich in dissolved minerals and supports a large population of chemoautotrophic bacteria. These bacteria use sulfur compounds, particularly
hydrogen sulfide, a chemical highly toxic to most known organisms, to produce organic material through the process of
chemosynthesis. The vents' impact on the living environment goes beyond the organisms that lives around them, as they act as a significant source of iron in the oceans, providing iron for the phytoplankton.
Biological communities The oldest confirmed record of a "modern" biological community related with a vent is the
Figueroa Sulfide, from the Early Jurassic of California. The ecosystem so formed is reliant upon the continued existence of the hydrothermal vent field as the primary source of energy, which differs from most surface life on Earth, which is based on
solar energy. However, although it is often said that these communities exist independently of the sun, some of the organisms are actually dependent upon oxygen produced by photosynthetic organisms, while others are
anaerobic. '') cluster around vents in the
Galapagos Rift. The chemosynthetic bacteria grow into a thick mat which attracts other organisms, such as
amphipods and
copepods, which graze upon the bacteria directly. Larger organisms, such as snails, shrimp, crabs,
tube worms, fish (especially
eelpout,
cutthroat eel,
Ophidiiformes and
Symphurus thermophilus), and octopuses (notably
Vulcanoctopus hydrothermalis), form a
food chain of predator and prey relationships above the primary consumers. The main families of organisms found around seafloor vents are
annelids,
gastropods, and crustaceans, with large
bivalves, vestimentiferan worms, and "eyeless" shrimp making up the bulk of nonmicrobial organisms.
Siboglinid tube worms, which may grow to over tall in the largest species, often form an important part of the community around a hydrothermal vent. They have no mouth or digestive tract, and like parasitic worms, absorb nutrients produced by the bacteria in their tissues. About 285 billion bacteria are found per ounce of tubeworm tissue. Tubeworms have red plumes which contain
hemoglobin. Hemoglobin combines with hydrogen sulfide and transfers it to the bacteria living inside the worm. In return, the bacteria nourish the worm with carbon compounds. Two of the species that inhabit a hydrothermal vent are
Tevnia jerichonana, and
Riftia pachyptila. One discovered community, dubbed "
Eel City", consists predominantly of the eel
Dysommina rugosa. Though eels are not uncommon, invertebrates typically dominate hydrothermal vents. Eel City is located near
Nafanua volcanic cone,
American Samoa. In 1993, already more than 100 gastropod species were known to occur in hydrothermal vents. Over 300 new species have been discovered at hydrothermal vents, many of them "sister species" to others found in geographically separated vent areas. It has been proposed that before the
North American Plate overrode the
mid-ocean ridge, there was a single biogeographic vent region found in the eastern Pacific. The subsequent barrier to travel began the evolutionary divergence of species in different locations. The examples of convergent evolution seen between distinct hydrothermal vents is seen as major support for the theory of natural selection and of evolution as a whole. Although life is very sparse at these depths, black smokers are the centers of entire
ecosystems. Sunlight is nonexistent, so many organisms, such as
archaea and
extremophiles, convert the heat,
methane, and
sulfur compounds provided by black smokers into energy through a process called
chemosynthesis. More complex life forms, such as
clams and
tubeworms, feed on these organisms. The organisms at the base of the
food chain also deposit minerals into the base of the black smoker, therefore completing the
life cycle. A species of phototrophic bacterium has been found living near a black smoker off the coast of
Mexico at a depth of . No sunlight penetrates that far into the waters. Instead, the bacteria, part of the
Chlorobiaceae family, use the faint glow from the black smoker for
photosynthesis. This is the first organism discovered in nature to exclusively use a light other than sunlight for photosynthesis. New and unusual species are constantly being discovered in the neighborhood of black smokers. The
Pompeii worm Alvinella pompejana, which is capable of withstanding temperatures up to , was found in the 1980s, and the
scaly-foot gastropod (
Chrysomallon squamiferum) was first found in 2001 during an expedition to the
Indian Ocean's
Kairei hydrothermal vent field. The latter uses iron sulfides (
pyrite and greigite) for the structure of its dermal
sclerites (hardened body parts), instead of
calcium carbonate. The extreme pressure of 2,500 m of water (approximately 25
megapascals or 250
atmospheres) is thought to play a role in stabilizing iron sulfide for biological purposes. This armor plating probably serves as a defense against the venomous
radula (teeth) of
predatory
snails in that community. In March 2017, researchers reported evidence of possibly the
oldest forms of life on Earth. Putative fossilized microorganisms were discovered in hydrothermal vent precipitates in the
Nuvvuagittuq Belt of
Quebec, Canada, that may have lived as
early as 4.280 billion years ago, not long after the oceans
formed 4.4 billion years ago, and not long after the
formation of the Earth 4.54 billion years ago.
), a few squat lobsters, and hundreds of bivalves (Bathymodiolus'') next to an extinct smoker
Animal-bacterial symbiosis anomurans and Vulcanolepas''-like stalked barnacles) near
East Scotia Ridge vents Hydrothermal vent ecosystems have enormous biomass and productivity, but this rests on the symbiotic relationships that have evolved at vents. Deep-sea hydrothermal vent ecosystems differ from their shallow-water and terrestrial hydrothermal counterparts due to the symbiosis that occurs between macroinvertebrate hosts and chemoautotrophic microbial symbionts in the former. Since sunlight does not reach deep-sea hydrothermal vents, organisms in deep-sea hydrothermal vents cannot obtain energy from the sun to perform photosynthesis. Instead, the microbial life found at hydrothermal vents is chemosynthetic; they fix carbon by using energy from chemicals such as sulfide, as opposed to light energy from the sun. In other words, the symbiont converts inorganic molecules (H2S, CO2, O) to organic molecules that the host then uses as nutrition. However, sulfide is an extremely toxic substance to most life on Earth. For this reason, scientists were astounded when they first found hydrothermal vents teeming with life in 1977. What was discovered was the ubiquitous symbiosis of chemoautotrophs living in (
endosymbiosis) the vent animals' gills; the reason why multicellular life is capable to survive the toxicity of vent systems. Scientists are therefore now studying how the microbial symbionts aid in sulfide detoxification (therefore allowing the host to survive the otherwise toxic conditions). Work on
microbiome function shows that host-associated microbiomes are also important in host development, nutrition, defense against predators, and detoxification. In return, the host provides the symbiont with chemicals required for chemosynthesis, such as carbon, sulfide, and oxygen. In the early stages of studying life at hydrothermal vents, there were differing theories regarding the mechanisms by which multicellular organisms were able to acquire nutrients from these environments, and how they were able to survive in such extreme conditions. In 1977, it was hypothesized that the chemoautotrophic bacteria at hydrothermal vents might be responsible for contributing to the diet of suspension-feeding bivalves. Finally, in 1981, it was understood that giant tubeworm nutrition acquisition occurred as a result of chemoautotrophic bacterial endosymbionts. As scientists continued to study life at hydrothermal vents, it was understood that symbiotic relationships between chemoautotrophs and macrofauna invertebrate species was ubiquitous. For instance, in 1983, clam gill tissue was confirmed to contain bacterial endosymbionts; in 1984 vent
bathymodiolid mussels and
vesicomyid clams were also found to carry endosymbionts. However, the mechanisms by which organisms acquire their symbionts differ, as do the metabolic relationships. For instance, tubeworms have no mouth and no gut, but they do have a "trophosome", which is where they deal with nutrition and where their endosymbionts are found. They also have a bright red plume, which they use to uptake compounds such as O, H2S, and CO2, which feed the endosymbionts in their trophosome. Remarkably, the tubeworms hemoglobin (which incidentally is the reason for the bright red color of the plume) is capable of carrying oxygen without interference or inhibition from sulfide, despite the fact that oxygen and sulfide are typically very reactive. In 2005, it was discovered that this is possible due to zinc ions that bind the hydrogen sulfide in the tubeworms hemoglobin, therefore preventing the sulfide from reacting with the oxygen. It also reduces the tubeworms tissue from exposure to the sulfide and provides the bacteria with the sulfide to perform chemoautotrophy. It has also been discovered that tubeworms can metabolize CO2 in two different ways, and can alternate between the two as needed as environmental conditions change. In 1988, research confirmed thiotrophic (sulfide-oxidizing) bacteria in
Alviniconcha hessleri, a large vent mollusk. In order to circumvent the toxicity of sulfide, mussels first convert it to thiosulfate before carrying it over to the symbionts. In the case of motile organisms such as alvinocarid shrimp, they must track oxic (oxygen-rich) / anoxic (oxygen-poor) environments as they fluctuate in the environment. Organisms living at the edge of hydrothermal vent fields, such as pectinid scallops, also carry endosymbionts in their gills, and as a result their bacterial density is low relative to organisms living nearer to the vent. However, the scallop's dependence on the microbial endosymbiont for obtaining their nutrition is therefore also lessened. Furthermore, not all host animals have endosymbionts; some have episymbionts—symbionts living on the animal as opposed to inside the animal. Shrimp found at vents in the Mid-Atlantic Ridge were once thought of as an exception to the necessity of symbiosis for macroinvertebrate survival at vents. That changed in 1988 when they were discovered to carry episymbionts. Since then, other organisms at vents have been found to carry episymbionts as well, such as Lepetodrilis fucensis. Furthermore, while some symbionts reduce sulfur compounds, others are known as "
methanotrophs" and reduce carbon compounds, namely methane. Bathmodiolid mussels are an example of a host that contains methanotrophic endosymbionts; however, the latter mostly occur in cold seeps as opposed to hydrothermal vents. While chemosynthesis occurring at the deep ocean allows organisms to live without sunlight in the immediate sense, they technically still rely on the sun for survival, since oxygen in the ocean is a byproduct of photosynthesis. However, if the sun were to suddenly disappear and photosynthesis ceased to occur on our planet, life at the deep-sea hydrothermal vents could continue for millennia (until the oxygen was depleted).
Theory of hydrothermal origin of life The chemical and thermal dynamics in hydrothermal vents makes such environments highly suitable thermodynamically for chemical evolution processes to take place. Therefore, thermal energy flux is a permanent agent and is hypothesized to have contributed to the evolution of the planet, including prebiotic chemistry. It has been proposed that
amino acid synthesis could have occurred deep in the Earth's crust and that these amino acids were subsequently shot up along with hydrothermal fluids into cooler waters, where lower temperatures and the presence of clay minerals would have fostered the formation of peptides and
protocells. This is an attractive hypothesis because of the abundance of CH4 (
methane) and NH3 (
ammonia) present in hydrothermal vent regions, a condition that was not provided by the Earth's primitive atmosphere. A major limitation to this hypothesis is the lack of stability of organic molecules at high temperatures, but some have suggested that life would have originated outside of the zones of highest temperature. There are numerous species of
extremophiles and other organisms currently living immediately around deep-sea vents, suggesting that this is indeed a possible scenario. Experimental research and computer modeling indicate that the surfaces of mineral particles inside hydrothermal vents have similar catalytic properties to enzymes and are able to create simple organic molecules, such as
methanol (CH3OH) and
formic acid (HCO2H), out of the dissolved CO2 in the water. Additionally, the discovery of
supercritical CO2 at some sites has been used to further support the theory of hydrothermal origin of life given that it can increase organic reaction rates. Its high solvation power and diffusion rate allow it to promote amino and
formic acid synthesis, as well as the synthesis of other organic compounds, polymers, and the four amino acids: alanine, arginine, aspartic acid, and glycine. In situ experiments have revealed the convergence of high N2 content and supercritical CO2 at some sites, as well as evidence for complex organic material (amino acids) within supercritical CO2 bubbles. Proponents of this theory for the origin of life also propose the presence of supercritical CO2 as a solution to the "water paradox" that pervades theories on the origin of life in aquatic settings. This paradox encompasses the fact that water is both required for life and will, in abundance, hydrolyze organic molecules and prevent
dehydration synthesis reactions necessary to chemical and biological evolution. Supercritical CO2, being hydrophobic, acts as a solvent that facilitates an environment conducive to dehydration synthesis. Therefore, it has been hypothesized that the presence of supercritical CO2 in
Hadean hydrothermal vents played an important role in the origin of life. One current theory is that the naturally occurring proton gradients at these deep sea vents supplemented the lack of phospholipid bilayer membranes and proton pumps in early organisms, allowing ion gradients to form despite the lack of cellular machinery and components present in modern cells. There is some discourse around this topic. It has been argued that the natural pH gradients of these vents playing a role in the origin of life is actually implausible. The counter argument relies, among other points, on what the author describes as the unlikelihood of the formation of machinery which produces energy from the pH gradients found in hydrothermal vents without/before the existence of genetic information. The ionic concentrations of hydrothermal vents differs from the intracellular fluid within the majority of life. It has instead been suggested that terrestrial freshwater environments are more likely to be an ideal environment for the formation of early cells. Meanwhile, proponents of the deep sea hydrothermal vent hypothesis suggest thermophoresis in mineral cavities to be an alternative compartment for polymerization of biopolymers. How thermophoresis within mineral cavities could promote coding and metabolism is unknown. Nick Lane suggests that nucleotide polymerization at high concentrations of nucleotides within self-replicating protocells, where "Molecular crowding and phosphorylation in such confined, high-energy protocells could potentially promote the polymerization of nucleotides to form RNA". Acetyl phosphate could possibly promote polymerization at mineral surfaces or at low water activity. A computational simulation shows that nucleotide concentration of nucleotide catalysis of "the energy currency pathway is favored, as energy is limiting; favoring this pathway feeds forward into a greater nucleotide synthesis". Fast nucleotide catalysis of fixation lowers nucleotide concentration as protocell growth and division is rapid which then leads to halving of nucleotide concentration, weak nucleotide catalysis of fixation promotes little to protocell growth and division. In biochemistry, reactions with CO2 and H2 produce precursors to biomolecules that are also produced from the acetyl-CoA pathway and
Krebs cycle which would support an origin of life at deep sea alkaline vents. Acetyl phosphate produced from the reactions are capable of phosphorylating ADP to ATP, with maximum synthesis occurring at high water activity and low concentrations of ions, the Hadean ocean likely had lower concentrations of ions than modern oceans. The concentrations of Mg2+ and Ca2+ at alkaline hydrothermal systems are lower than those at the ocean. The high concentration of potassium within most life forms could be readily explained that protocells might have evolved sodium-hydrogen antiporters to pump out Na+ as prebiotic lipid membranes are less permeable to Na+ than H+. If cells originated at these environments, they would have been autotrophs with a Wood-Ljungdahl pathway and incomplete reverse Krebs cycle. Mathematical modelling of organic synthesis of carboxylic acids to lipids, nucleotides, amino acids, and sugars, and polymerization reactions are favorable at alkaline hydrothermal vents.
The Deep Hot Biosphere At the beginning of his 1992 paper
The Deep Hot Biosphere,
Thomas Gold referred to
ocean vents in support of his theory that the lower levels of the earth are rich in living biological material that finds its way to the surface. He further expanded his ideas in the book
The Deep Hot Biosphere. An article on
abiogenic hydrocarbon production in the February 2008 issue of
Science journal used data from experiments at the
Lost City hydrothermal field to report how the abiotic synthesis of low molecular mass hydrocarbons from mantle derived carbon dioxide may occur in the presence of ultramafic rocks, water, and moderate amounts of heat. ==Discovery and exploration==