No Sunlight, No Photosynthesis — So How Is the Deep Sea Creating Oxygen?

For generations, we’ve been taught a simple rule: oxygen comes from sunlight. Plants, algae, and microscopic phytoplankton use light to power photosynthesis, releasing the oxygen that fills our atmosphere and oceans. No sunlight, no oxygen. It felt like one of those scientific truths that didn’t need revisiting.

But more than 13,000 feet below the surface of the Pacific Ocean, in total darkness, researchers have found evidence that oxygen may be forming on its own.

And it’s not coming from living organisms.


A Discovery in the Dark

The finding was reported in a peer-reviewed study published in Nature Geoscience, led by Professor Andrew Sweetman of the Scottish Association for Marine Science, alongside chemist Franz Geiger of Northwestern University. The team was conducting long-term measurements in the Clarion-Clipperton Zone, a vast abyssal region between Hawaii and Mexico known for its rich deposits of polymetallic nodules.

These nodules — small, rock-like lumps scattered across the seabed — contain manganese, nickel, cobalt, copper, and lithium. They’ve drawn global attention because those metals are critical for batteries and electric vehicles. But while studying oxygen levels near these deposits, researchers noticed something unexpected.

Instead of oxygen declining over time, as it typically does in deep waters where organisms consume it, sensors showed oxygen increasing. At first, the team suspected equipment failure. For decades, deep-sea research had consistently shown oxygen being used up, not produced.

Repeated measurements told the same story. Oxygen was being generated on the seafloor itself.

Natural “Geobatteries” Beneath the Sea

The explanation appears to lie within the nodules.

In laboratory analyses, researchers found that individual polymetallic nodules can generate measurable electrical voltages, up to 1 volt. When multiple nodules cluster together, their electrical potentials can combine, much like batteries connected in series.

Under normal conditions, splitting water into hydrogen and oxygen through electrolysis requires about 1.23 volts. While the voltages observed on the seafloor are slightly lower, the researchers suggest that the unique high-pressure, saline conditions of the deep ocean may enable electrochemical pathways that lower the energy threshold for oxygen formation.

Franz Geiger described these formations as natural “geobatteries.” Through corrosion and chemical interactions in seawater, metals such as manganese and iron can create small electric currents. That electricity may drive reactions capable of splitting water molecules, releasing oxygen in complete darkness.

Electrolysis is familiar in industrial and laboratory settings. What makes this discovery striking is the possibility that it may occur naturally, miles beneath the ocean surface.

Why This Changes the Oxygen Story

In much of modern science, oxygen production has been almost entirely tied to biology. Photosynthesis was seen as the dominant source. If the deep sea can generate oxygen without sunlight or living organisms, that assumption needs to be expanded.

This matters for understanding Earth’s past. Before photosynthetic life became widespread billions of years ago, Earth’s oceans were largely oxygen-poor. If abiotic processes like this were occurring even at small scales, they may have created localized oxygen pockets that influenced early microbial evolution.

The implications extend beyond Earth as well. Research exploring the formation of abiotic oxygen on icy moons and exoplanets suggests that similar electrochemical processes could occur in subsurface oceans elsewhere in the solar system. Moons such as Europa and Enceladus contain salty oceans beneath thick ice shells. If mineral-driven oxygen production is possible there, environments without sunlight might still support aerobic chemistry.

Oxygen, in other words, may not always signal photosynthesis.

Ongoing Questions and Scientific Debate

The discovery has sparked careful debate within the scientific community. Some researchers question whether the voltages recorded are sufficient to sustain meaningful oxygen production over long periods. Others are examining the thermodynamics of electrolysis under extreme deep-sea pressures.

Follow-up studies are underway to verify reaction rates, quantify oxygen output, and determine the extent of this process across the ocean floor.

At the same time, the finding intersects with growing interest in deep-sea mining. The same nodules that potentially generate oxygen also contain metals essential for renewable energy technology. Scientists are now considering whether removing these deposits could disrupt chemical processes and ecosystems that are not yet fully understood.



Rethinking the Deep Ocean

The deep sea is often imagined as quiet and chemically static — a place where life survives on nutrients drifting down from above. This research suggests a more active picture. The seabed itself may host chemical systems capable of producing oxygen in the absence of light.

It challenges a rule that once seemed absolute.

For decades, sunlight was considered the engine behind oxygen on Earth. Now, evidence from the darkest parts of the planet hints that chemistry alone may sometimes do the job.

And that realization reshapes how we think about our oceans — and possibly about life beyond them.