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| No sunlight, no photosynthesis — so how is the deep sea creating oxygen? | | | Dr Vijay Garg
For decades, scientists believed that nearly all oxygen on Earth is produced through photosynthesis — a process driven by sunlight and carried out by plants, algae, and cyanobacteria. According to this understanding, the deep ocean, where sunlight cannot penetrate, should be an oxygen-consuming environment rather than an oxygen-producing one.
However, recent deep-sea research has revealed surprising processes that may generate oxygen even in complete darkness. These discoveries are reshaping our understanding of ocean chemistry, deep-sea ecosystems, and even the origins of life.
Why Sunlight Matters for Oxygen Production
Photosynthesis uses sunlight to convert carbon dioxide and water into glucose and oxygen. This process occurs in surface waters where light can reach.
Below about 200 meters — known as the aphotic zone — sunlight disappears. For years, scientists assumed oxygen found at these depths was transported from surface waters through ocean circulation.
But emerging research suggests that oxygen may also be produced in situ in the deep sea.
Discovery of “Dark Oxygen” Production
In 2024, a team of researchers led by Andrew Sweetman reported unexpected oxygen increases on the seafloor of the Pacific Ocean. Their findings suggested oxygen was being generated without light — a phenomenon now often called dark oxygen production.
Sensors deployed on deep seabeds recorded oxygen levels rising even in sealed experimental chambers, ruling out photosynthesis or mixing from upper waters.
How Can Oxygen Form Without Sunlight?
Scientists are exploring several mechanisms that could explain this surprising phenomenon:
1. Electrochemical Reactions from Metallic Nodules
Polymetallic nodules — rock-like deposits rich in manganese, nickel, cobalt, and iron — cover vast areas of the deep seabed.
These nodules can act like natural batteries:
Seawater acts as an electrolyte
Mineral surfaces create electrical gradients
Electrochemical reactions split water molecules
Oxygen is released as a byproduct
If confirmed at scale, this process could represent a previously unknown oxygen source.
2. Radiolysis: Water Splitting by Natural Radiation
Natural radioactive decay in seafloor sediments may split water molecules into hydrogen and oxygen. This process, known as radiolysis, occurs without light and has been observed in deep underground environments.
Although production rates are small, they may be sufficient to support microbial ecosystems.
3. Microbial Chemical Processes
Certain microbes can generate oxygen through chemical reactions involving nitrogen and sulfur compounds.
One example is nitrite-driven anaerobic methane oxidation, in which microbes convert nitrite into nitrogen gas and oxygen — allowing methane to be consumed even in oxygen-poor environments.
These microbial pathways could sustain unique deep-sea life.
Why This Discovery Matters
Supporting Deep-Sea Ecosystems
Dark oxygen production could help sustain life in isolated, oxygen-limited environments, supporting microbes, worms, crustaceans, and other deep-sea organisms.
Rethinking Ocean Oxygen Cycles
If deep-sea oxygen generation is widespread, scientists may need to revise models of global oxygen cycling and carbon storage.
Implications for Climate Science
Deep ocean chemistry plays a crucial role in carbon sequestration and climate regulation. Understanding oxygen sources improves climate predictions.
Clues About Life Beyond Earth
Oxygen production without sunlight expands the possibilities for life in dark environments such as subsurface oceans on icy moons like Europa and Enceladus.
A Frontier Still Being Explored
Scientists caution that dark oxygen production is still under investigation. More measurements across different ocean basins are needed to determine how widespread and significant the process is.
Yet this discovery challenges a fundamental assumption: that sunlight is required for oxygen generation.
The deep ocean — long thought to be a silent consumer of oxygen — may also be quietly producing it.
Conclusion
The possibility of oxygen being created in total darkness reveals how little we still know about Earth’s oceans. From electrochemical reactions on the seabed to microbial chemistry and natural radiation, the deep sea continues to surprise scientists.
Even without sunlight, life finds a way — and the ocean still holds many secrets waiting to be uncovered. |
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