In order for life to emerge on Earth, a wide and diverse range of organic compounds was necessary. The contribution from earthly processes is unclear although it is thought that outgassing and hydrothermal fluid-rock interactions have played major roles within the process. Here, we can take a deeper look into hydrothermal vents, and their preassumed roles in the origin of life through biological and abiogenic processes.
Hydrothermal Vents and their Importance
Hydrothermal vents are geochemically reactive habitats found in the seabed mostly near volcanically active areas. These vents, sourced by mid-ocean ridge volcanism, maintain rich ecosystems [1,2].
Hydrothermal vents have been hallmarks of our planet since the Hadean. As a result of that, their history lets us take a better look at the history of life on our world. Although the chances of preservation of vent fossils are visibly low, recent fossil findings from ancient vent environments have given us priceless insights into the history of life at hydrothermal vents [3,4]. While these fossils are amongst the most ancient contenders for visible evidence of life on Earth, a range of additional fossil findings imply that vent habitats were already exploited by microbes during Precambrian [4]. It is assumed that the first metazoans probably appeared within vents during the Cambrian, and by the Ordovician-Silurian, hydrothermal vent systems in the deep sea were colonized by large quantities and sizes of mollusc, tubeworm and brachiopod taxa, which suggest that these animals were efficiently adapted to this setting [4].
As we know, carbon is the key element of life. Keeping this in mind, it plays a crucial role in understanding the origins and the primordial evolution of life.
Almost 50 years ago, submarine hydrothermal vents were discovered, changing the views on the source of life’s reduced carbon [5]. The hydrothermal vents seemed to reveal the previously unknown chemistry upon the earth. This discovery had a sudden impact on hypotheses about the origin of life, mostly because it was assessed that these vent systems were chemically reactive, thus maintaining suitable conditions for life to emerge through prebiotic synthesis [1,2,4]. The oldest putative fossiliferous vent deposit within the Nuvvuagittuq Supracrustal Belt, Canada, occurs as hematite filaments and tubes in jasper-carbonate banded iron formations [3,4]. These fossils, if biological in origin, might have resemblances to modern descendants; however, if abiotic, they could indicate complex prebiotic forms on early Earth. They are also considered to have formed as a result of hydrothermal activity [1–4].
Scientists have found noticeable similarities between the chemistry of the H2-CO2 oxidation–reduction couple that remain present in both hydrothermal vents and the core energy metabolic reactions of some modern prokaryotic autotrophs [5]. The biochemistry of these autotrophs possesses clues about the reaction types that might have generated the chemistry of life [2,5].
There are two kinds of hydrothermal vents: the Black Smoker type (approximately 360°C) and the Lost City type (approximately 50-90°C) [1], [2].
Black Smoker Type
These vents are located right above the magma chambers that are found 1-3 kilometers beneath the sea floor. Beneath the seabed, during its circulation from the ocean floor, the sea water comes into close contact with the magma chamber before moving through the crust to re-emerge at the vents [1,6].
![Figure 1: Black Smoker type vent at 2,980m depth [7].](https://gencomugen.com/wp-content/uploads/Resim1-23.png)
It’s observed that black smoker chimneys emit hot and chemically modified ocean water (usually up to 405°C). Black smokers usually have acidic (pH 2-3) discharge fluids. These fluids are rich in dissolved transition metals such as Fe(II) and Mn(II). Since they’re fueled by volcanoes, black smoker fluids typically contain high levels of magmatic CO2, H2S and dissolved H2 with various amounts of CH4 which is formed through biogenic and abiogenic processes [5,8,9]. These dissolved gases and metals fuel the microbial communities that play a role as the base of the food chain in such ecosystems [6].
Some of the black smoker chimneys have a jagged appearance, which reflects the fact that the chimney walls are encrusted in dense communities of tube worms, scale worms and such [1,6].
Lost City Systems
Off-axis vents, also known as the lost city systems, are characterized by carbonate chimneys that rise 60 meters above the ultramafic sea floor and are generally located several kilometers away from the spreading zone [2]. Scientists believe that these systems might lead us to understand the origins of life. Ultramafic underpinnings of the lost city systems have a similar chemical composition to lavas that erupted into the primordial oceans on early Earth. As a consequence of this, the lost city hydrothermal field gives us clues about past mantle geochemistry and confirms a better understanding of the chemical constraints that existed during the evolutionary transition from geochemical to biochemical processes [2,4].

Fluid circulation within the crust is driven by convection that dissipates heat from the underlying mantle rocks, and most likely, by exothermic chemical reactions between the circulating fluids and host rocks. These rocks have different compositions compared with those of underwater volcanoes since they are dominated by a mineral called olivine, which is abundant in iron and magnesium [1,2,5,11,12].
These factors altogether result in a highly alkaline effluent and a combination of extreme conditions which have not been observed before in the marine environment. When contemplating the biochemical origins of life, alkaline pH is a crucial feature of vents. In the Hadean, alkaline vents are thought to act as electrochemical flow reactors through a labyrinth of interconnected micropores with thin inorganic walls containing catalytic minerals since oxygen was absent. Across these thin walls, the difference in pH levels assembled natural proton gradients with equivalent magnitude and polarity needed for carbon fixation in extant bacteria and archaea [2,8].
An undeniable proportion of the seafloor consisting of ultramafic rocks has also been identified- and is highly similar to those that host the lost city hydrothermal field. These ultramafic rocks are, in fact, sites of an important set of geochemical reactions named serpentinization. They’ve been producing geological H2 for as long as there has been water on Earth.
What is Serpentinization?
Serpentinization is the hydration of olivine and orthopyroxene minerals -which are the main components of ultramafic rocks- and creates a reducing chemical environment distinguished by high H2 concentrations that are well suited to abiotic hydrocarbon production[1], [3], [11]. Serpentinization occurs both beneath the hot and acidic black smokers and within the cooler and alkaline lost city hydrothermal field. However, even though both types of vent systems would have delivered a pH scale that was similar to the Hadean Ocean, lower temperatures within off-axis vents would maintain more benign conditions for sustained abiotic systems and the accumulation of reduced carbon compounds. At lost city systems, the seawater infests the warm to hot (100 to 400°C) oceanic crust through cracks where the chemical reactions of serpentinization happen [2].
The applicable sea-water components for the serpentinization reaction are H2O and CO2 (dissolved as HCO3−). The relevant crustal components are Fe2+ containing rocks. At Lost City, this rock contains mainly the mineral olivine. It is reported that during serpentinization, one cubic meter of olivine can deliver approximately 500 moles of H2. Seismic data implies that the fluids beneath off-axis vents drain to depths of 500 meters (or even deeper in some cases) beneath the sea floor at fairly high temperatures (150-200°C). Based on magnetic anomaly information, the crust beneath Lost City is 1-2 million years old, and it is thought that the rocks which are nearly 500 meters to 1 kilometer beneath the sea floor reach temperatures of ∼300°C. Within these conditions, Fe2+ in the rocks reduces H2O to produce Fe3+, H2 and hydrocarbons [4].
In a research made in 2008, reports showed that hydrothermally changed and unchanged mantle rocks could hold numerous carbon compounds, including graphite, CH4, and CO2 [11]. This reveal indicates that hydrocarbon production at Lost City is not a biological, but a geochemical process. The study, therefore, implies serpentinization in abiogenic carbon reduction, which could be highly applicable in an origin-of-life context [11].

Even though the science world has come a long way in understanding the hydrothermal vents and their roles in the origins of life and reduced carbon through biogenic and abiogenic processes, numerous vent sites are still waiting to be explored and analysed. Here is a map of Hydrothermal Vent sites from around the world.
References:
- W. Martin, J. Baross, D. Kelley, and M. J. Russell, “Hydrothermal vents and the origin of life,” Nature Reviews Microbiology 2008 6:11, vol. 6, no. 11, pp. 805–814, Sep. 2008, doi: 10.1038/nrmicro1991.
- V. Sojo, B. Herschy, A. Whicher, E. Camprubí, and N. Lane, “The Origin of Life in Alkaline Hydrothermal Vents,” https://home.liebertpub.com/ast, vol. 16, no. 2, pp. 181–197, Feb. 2016, doi: 10.1089/AST.2015.1406.
- D. Papineau et al., “Metabolically diverse primordial microbial communities in Earth’s oldest seafloor-hydrothermal jasper,” Sci Adv, vol. 8, no. 15, p. 2296, Apr. 2022, doi: 10.1126/SCIADV.ABM2296/SUPPL_FILE/SCIADV.ABM2296_MOVIES_S1_TO_S8.ZIP.
- M. N. Georgieva, C. T. S. Little, V. V. Maslennikov, A. G. Glover, N. R. Ayupova, and R. J. Herrington, “The history of life at hydrothermal vents,” Earth Sci Rev, vol. 217, p. 103602, Jun. 2021, doi: 10.1016/J.EARSCIREV.2021.103602.
- B. Rasmussen and J. R. Muhling, “Organic carbon generation in 3.5-billion-year-old basalt-hosted seafloor hydrothermal vent systems,” Sci Adv, vol. 9, no. 5, Feb. 2023, doi: 10.1126/SCIADV.ADD7925/ASSET/CEEE756F-CD47-4195-A058-D150EC9667D3/ASSETS/IMAGES/LARGE/SCIADV.ADD7925-F9.JPG.
- G. J. Dick, “The microbiomes of deep-sea hydrothermal vents: distributed globally, shaped locally,” Nature Reviews Microbiology 2019 17:5, vol. 17, no. 5, pp. 271–283, Mar. 2019, doi: 10.1038/s41579-019-0160-2.
- “13 Black smoker hydrothermal vent at 2,980m depth, Mid-Atlantic Ridge. | Download Scientific Diagram.” https://www.researchgate.net/figure/Black-smoker-hydrothermal-vent-at-2-980m-depth-Mid-Atlantic-Ridge_fig16_284419320 (accessed May 04, 2023).
- Y. Yamashita, Y. Mori, and H. Ogawa, “Hydrothermal-derived black carbon as a source of recalcitrant dissolved organic carbon in the ocean,” Sci Adv, vol. 9, no. 6, p. eade3807, Feb. 2023, doi: 10.1126/SCIADV.ADE3807/SUPPL_FILE/SCIADV.ADE3807_DATA_S1_AND_S2.ZIP.
- D. Trail and T. M. McCollom, “Relatively oxidized fluids fed Earth’s earliest hydrothermal systems,” Science (1979), vol. 379, no. 6632, pp. 582–585, Feb. 2023, doi: 10.1126/SCIENCE.ADC8751/SUPPL_FILE/SCIENCE.ADC8751_DATA_S1.ZIP.
- “Deep sea mining could destroy underwater Lost City, scientists warn | Climate News | Sky News.” https://news.sky.com/story/deep-sea-mining-could-destroy-underwater-lost-city-scientists-warn-11277837 (accessed May 04, 2023).
- G. Proskurowski et al., “Abiogenic hydrocarbon production at lost city hydrothermal field,” Science (1979), vol. 319, no. 5863, pp. 604–607, Feb. 2008, doi: 10.1126/SCIENCE.1151194/SUPPL_FILE/PROSKUROWSKI.SOM.PDF.
- B. Herschy et al., “An Origin-of-Life Reactor to Simulate Alkaline Hydrothermal Vents,” Journal of Molecular Evolution 2014 79:5, vol. 79, no. 5, pp. 213–227, Nov. 2014, doi: 10.1007/S00239-014-9658-4.
- “File:Hydrothermal vents map.svg – Wikimedia Commons.” https://commons.wikimedia.org/wiki/File:Hydrothermal_vents_map.svg (accessed May 04, 2023)
Figure References:
- “13 Black smoker hydrothermal vent at 2,980m depth, Mid-Atlantic Ridge. | Download Scientific Diagram.” https://www.researchgate.net/figure/Black-smoker-hydrothermal-vent-at-2-980m-depth-Mid-Atlantic-Ridge_fig16_284419320 (accessed May 04, 2023).
- “Deep sea mining could destroy underwater Lost City, scientists warn | Climate News | Sky News.” https://news.sky.com/story/deep-sea-mining-could-destroy-underwater-lost-city-scientists-warn-11277837 (accessed May 04, 2023).
- “File:Hydrothermal vents map.svg – Wikimedia Commons.” https://commons.wikimedia.org/wiki/File:Hydrothermal_vents_map.svg (accessed May 04, 2023).
Inspector: Celal Can VAROL