Hot spring
In areas of high volcanic activity, magma (molten rock) may be present at shallow depths in the Earth's crust.
This generally takes place along faults, where shattered rock beds provide easy paths for water to circulate to greater depths.
This leads to a runaway condition in which a sizable amount of water and steam are forcibly ejected from the hot spring as the cistern is emptied.
Here the groundwater originates as rain and snow (meteoric water) falling on the nearby mountains, which penetrates a particular formation (Hollis Quartzite) to a depth of 3,000 feet (910 m) and is heated by the normal geothermal gradient.
[30] This process is slow enough that geyserite is not all deposited immediately around the vent, but tends to build up a low, broad platform for some distance around the spring opening.
[31] When the fluids reach the surface, CO2 is rapidly lost and carbonate minerals precipitate as travertine, so that bicarbonate hot springs tend to form high-relief structures around their openings.
[38] For example, in a bicarbonate hot spring, the community of organisms immediately around the vent is dominated by filamentous thermophilic bacteria, such as Aquifex and other Aquificales, that oxidize sulfide and hydrogen to obtain energy for their life processes.
Further from the vent, where water temperatures have dropped below 60 °C (140 °F), the surface is covered with microbial mats 1 centimetre (0.39 in) thick that are dominated by cyanobacteria, such as Spirulina, Oscillatoria, and Synechococcus,[39] and green sulfur bacteria such as Chloroflexus.
Still further from the vent, where temperatures drop below 45 °C (113 °F), conditions are favorable for a complex community of microorganisms that includes Spirulina, Calothrix, diatoms and other single-celled eukaryotes, and grazing insects and protozoans.
[38] Alkali chloride hot springs show a similar succession of communities of organisms, with various thermophilic bacteria and archaea in the hottest parts of the vent.
Acid sulfate hot springs show a somewhat different succession of microorganisms, dominated by acid-tolerant algae (such as members of Cyanidiophyceae), fungi, and diatoms.
[44] The ionic composition and concentration of hot springs (K, B, Zn, P, O, S, C, Mn, N, and H) are identical to the cytoplasm of modern cells and possibly to those of the LUCA or early cellular life according to phylogenomic analysis.
Where continuous exposure to sunlight leads to the development of photosynthetic properties and later colonize on land and life at hydrothermal vents is suggested to be a later adaptation.
They show that fatty acids self-assemble into membranous structures and encapsulate synthesized biomolecules during exposure to UV light and multiple wet-dry cycles at slightly alkaline or acidic hot springs, which would not happen at saltwater conditions as the high concentrations of ionic solutes there would inhibit the formation of membranous structures.
[46][47][48] David Deamer and Bruce Damer note that these hypothesized prebiotic environments resemble Charles Darwin's imagined "warm little pond".
[46] If life did not emerge at deep sea hydrothermal vents, rather at terrestrial pools, extraterrestrial quinones transported to the environment would generate redox reactions conducive to proton gradients.
Without continuous wet-dry cycling to maintain stability of primitive proteins for membrane transport and other biological macromolecules, they would go through hydrolysis in an aquatic environment.
[46] Scientists discovered a 3.48 billion year old geyserite that seemingly preserved fossilized microbial life, stromatolites, and biosignatures.
[50] Like alkaline hydrothermal vents, the Hakuba Happo hot spring goes through serpentinization, suggesting methanogenic microbial life possibly originated in similar habitats.
[42] Carbonaceous meteors during the Late Heavy Bombardment would not have caused cratering on Earth as they would produce fragments upon atmospheric entry.
[54] Metabolic pathways have not yet been demonstrated at these environments,[52] but the development of proton gradients might have been generated by redox reactions coupled to meteoric quinones or protocell growth.
[60] Hot spring baths (onsen) have been in use in Japan for at least two thousand years, traditionally for cleanliness and relaxation, but increasingly for their therapeutic value.
[62] Because of both the folklore and the claimed medical value attributed to some hot springs, they are often popular tourist destinations, and locations for rehabilitation clinics for those with disabilities.
