Dark oxygen explained

Dark oxygen production (DOP) refers to the generation of molecular oxygen (O₂) through processes that do not involve light-dependent oxygenic photosynthesis. While the majority of Earth's oxygen is produced by plants and photosynthetically active microorganisms via photosynthesis, DOP occurs via a variety of abiotic and biotic processes and may support aerobic metabolism in dark, anoxic environments.

Abiotic DOP

Abiotic DOP can occur through several mechanisms, such as:

• Water radiolysis: This process typically takes place in dark geological ecosystems, such as aquifers, where the decay of radioactive elements in surrounding rock leads to the breakdown of water molecules, producing O_2.^[1]
• Oxidation of surface-bound radicals: On silicon-bearing minerals like quartz, surface-bound radicals can undergo oxidation, contributing to O₂ production.^{[2] [3] [4]}

In addition to direct O₂ formation, these processes often produce reactive oxygen species (ROS), such as hydroxyl radicals (OH^•), superoxide (O₂^•-), and hydrogen peroxide (H₂O₂). These ROS can be converted into O₂ and water either biotically, through enzymes like superoxide dismutase and catalase, or abiotically, via reactions with ferrous iron and other reduced metals.^{[5] [6]}

Biotic DOP

Biotic DOP is performed by microorganisms through distinct microbial processes, including:

• Chlorite dismutation: This involves the dismutation of chlorite (ClO₂^-) into O₂ and chloride ions.^[7]
• Nitric oxide dismutation: This involves the dismutation of nitric oxide (NO) into O₂ and dinitrogen gas (N₂) or nitrous oxide (N₂O). ^{[8] [9] [10]}
• Water lysis via methanobactins: Methanobactins can lyse water molecules to produce O_2.^[11]

These processes enable microbial communities to sustain aerobic metabolism in environments that lack oxygen.

Experimental evidence

Recent studies have provided compelling evidence for DOP in various geological and subsurface environments:

• Groundwater ecosystems: Dissolved oxygen concentrations have been measured in old groundwaters previously assumed to be anoxic. The presence of O­₂ is attributed to microbial communities capable of producing dark oxygen and water radiolysis. Metagenomic analyses and oxygen isotope studies further support local oxygen generation rather than atmospheric mixing.^[12]
• Seafloor environments: Evidence for DOP has also been suggested on the deep ocean floor, particularly in regions rich in polymetallic nodules. These nodules are proposed to generate sufficient electrical current to drive seawater electrolysis, resulting in O­₂ production.^[13]

Implications

Despite its diverse pathways, DOP has traditionally been considered negligible in Earth's systems. Recent evidence suggests that O₂ is produced and consumed in dark, apparently anoxic environments on a much larger scale than previously thought, with implications for global biogeochemical cycles.^{[14] [15]}

Notes and References

1. Das . Soumya . 2013 . Critical Review of Water Radiolysis Processes, Dissociation Products, and Possible Impacts on the Local Environment: A Geochemist . Australian Journal of Chemistry . en . 66 . 5 . 522 . 10.1071/CH13012 . 0004-9425.
2. He . Hongping . Wu . Xiao . Xian . Haiyang . Zhu . Jianxi . Yang . Yiping . Lv . Ying . Li . Yiliang . Konhauser . Kurt O. . 2021-11-16 . An abiotic source of Archean hydrogen peroxide and oxygen that pre-dates oxygenic photosynthesis . Nature Communications . en . 12 . 1 . 6611 . 10.1038/s41467-021-26916-2 . 2041-1723 . 8595356 . 34785682. 2021NatCo..12.6611H .
3. He . Hongping . Wu . Xiao . Zhu . Jianxi . Lin . Mang . Lv . Ying . Xian . Haiyang . Yang . Yiping . Lin . Xiaoju . Li . Shan . Li . Yiliang . Teng . H. Henry . Thiemens . Mark H. . 2023-03-28 . A mineral-based origin of Earth's initial hydrogen peroxide and molecular oxygen . Proceedings of the National Academy of Sciences . en . 120 . 13 . e2221984120 . 10.1073/pnas.2221984120 . 0027-8424 . 10068795 . 36940327. 2023PNAS..12021984H .
4. Stone . Jordan . Edgar . John O. . Gould . Jamie A. . Telling . Jon . 2022-08-08 . Tectonically-driven oxidant production in the hot biosphere . Nature Communications . en . 13 . 1 . 4529 . 10.1038/s41467-022-32129-y . 2041-1723 . 9360021 . 35941147. 2022NatCo..13.4529S .
5. Sutherland . Kevin M. . Hemingway . Jordon D. . Johnston . David T. . May 2022 . The influence of reactive oxygen species on "respiration" isotope effects . Geochimica et Cosmochimica Acta . en . 324 . 86–101 . 10.1016/j.gca.2022.02.033. 2022GeCoA.324...86S .
6. Xu . Jie . Sahai . Nita . Eggleston . Carrick M. . Schoonen . Martin A.A. . February 2013 . Reactive oxygen species at the oxide/water interface: Formation mechanisms and implications for prebiotic chemistry and the origin of life . Earth and Planetary Science Letters . en . 363 . 156–167 . 10.1016/j.epsl.2012.12.008. 2013E&PSL.363..156X .
7. Xu . Jianlin . Logan . Bruce E. . August 2003 . Measurement of chlorite dismutase activities in perchlorate respiring bacteria . Journal of Microbiological Methods . en . 54 . 2 . 239–247 . 10.1016/S0167-7012(03)00058-7. 12782379 .
8. Ettwig . Katharina F. . Speth . Daan R. . Reimann . Joachim . Wu . Ming L. . Jetten . Mike S. M. . Keltjens . Jan T. . 2012 . Bacterial oxygen production in the dark . Frontiers in Microbiology . 3 . 273 . 10.3389/fmicb.2012.00273 . free . 1664-302X . 3413370 . 22891064.
9. Kraft . Beate . Jehmlich . Nico . Larsen . Morten . Bristow . Laura A. . Könneke . Martin . Thamdrup . Bo . Canfield . Donald E. . 2022-01-07 . Oxygen and nitrogen production by an ammonia-oxidizing archaeon . Science . en . 375 . 6576 . 97–100 . 10.1126/science.abe6733 . 34990242 . 2022Sci...375...97K . 0036-8075.
10. Murali . Ranjani . Pace . Laura A. . Sanford . Robert A. . Ward . L. M. . Lynes . Mackenzie M. . Hatzenpichler . Roland . Lingappa . Usha F. . Fischer . Woodward W. . Gennis . Robert B. . Hemp . James . 2024-06-25 . Diversity and evolution of nitric oxide reduction in bacteria and archaea . Proceedings of the National Academy of Sciences . en . 121 . 26 . e2316422121 . 10.1073/pnas.2316422121 . 0027-8424 . 11214002 . 38900790. December 20, 2024 . 2024PNAS..12116422M .
11. Dershwitz . Philip . Bandow . Nathan L. . Yang . Junwon . Semrau . Jeremy D. . McEllistrem . Marcus T. . Heinze . Rafael A. . Fonseca . Matheus . Ledesma . Joshua C. . Jennett . Jacob R. . DiSpirito . Ana M. . Athwal . Navjot S. . Hargrove . Mark S. . Bobik . Thomas A. . Zischka . Hans . DiSpirito . Alan A. . 2021-06-25 . Parales . Rebecca E. . Oxygen Generation via Water Splitting by a Novel Biogenic Metal Ion-Binding Compound . Applied and Environmental Microbiology . en . 87 . 14 . e0028621 . 10.1128/AEM.00286-21 . 0099-2240 . 8231713 . 33962982. 2021ApEnM..87E.286D .
12. Ruff . S. Emil . Humez . Pauline . de Angelis . Isabella Hrabe . Diao . Muhe . Nightingale . Michael . Cho . Sara . Connors . Liam . Kuloyo . Olukayode O. . Seltzer . Alan . Bowman . Samuel . Wankel . Scott D. . McClain . Cynthia N. . Mayer . Bernhard . Strous . Marc . 2023-06-13 . Hydrogen and dark oxygen drive microbial productivity in diverse groundwater ecosystems . Nature Communications . en . 14 . 1 . 3194 . 10.1038/s41467-023-38523-4 . 2041-1723 . 10264387 . 37311764. 2023NatCo..14.3194R .
13. Sweetman . Andrew K. . Smith . Alycia J. . de Jonge . Danielle S. W. . Hahn . Tobias . Schroedl . Peter . Silverstein . Michael . Andrade . Claire . Edwards . R. Lawrence . Lough . Alastair J. M. . Woulds . Clare . Homoky . William B. . Koschinsky . Andrea . Fuchs . Sebastian . Kuhn . Thomas . Geiger . Franz . August 2024 . Evidence of dark oxygen production at the abyssal seafloor . Nature Geoscience . en . 17 . 8 . 737–739 . 10.1038/s41561-024-01480-8 . 1752-0894.
14. Sweetman . Andrew K. . Smith . Alycia J. . de Jonge . Danielle S. W. . Hahn . Tobias . Schroedl . Peter . Silverstein . Michael . Andrade . Claire . Edwards . R. Lawrence . Lough . Alastair J. M. . Woulds . Clare . Homoky . William B. . Koschinsky . Andrea . Fuchs . Sebastian . Kuhn . Thomas . Geiger . Franz . August 2024 . Evidence of dark oxygen production at the abyssal seafloor . Nature Geoscience . en . 17 . 8 . 737–739 . 10.1038/s41561-024-01480-8 . 1752-0894.
15. Ruff . S. Emil . Humez . Pauline . de Angelis . Isabella Hrabe . Diao . Muhe . Nightingale . Michael . Cho . Sara . Connors . Liam . Kuloyo . Olukayode O. . Seltzer . Alan . Bowman . Samuel . Wankel . Scott D. . McClain . Cynthia N. . Mayer . Bernhard . Strous . Marc . 2023-06-13 . Hydrogen and dark oxygen drive microbial productivity in diverse groundwater ecosystems . Nature Communications . en . 14 . 1 . 10.1038/s41467-023-38523-4 . 2041-1723 . 10264387 . 37311764.