Since time immemorial, oxygen is said to be equally crucial for marine life as well as for land. It’s undoubtedly acknowledged that a high percentage of the source of oxygen is the ocean. Oxygen in the ocean is vital since most biological activities are performed with its aid. Processes like respiration, where marine organisms such as fish require oxygen to produce energy through cellular respiration for breathing and survival, and most importantly photosynthesis, where organisms like phytoplankton—microscopic algae—utilize sunlight to convert carbon dioxide and water into oxygen and organic matter consumed by other organisms, contribute to the entire ecosystem.

This indicates that oxygen underwater is of high value and marine life heavily relies on it for survival, evidence of which can be seen in dead zones where there is zero survival of marine life due to oxygen depletion.

Light Penetration and Ocean Zones

A major source of oxygen in the ocean is sunlight. According to the National Oceanic and Atmospheric Administration, sunlight can reach up to 1,000 meters deep in the ocean under ideal conditions, but meaningful light rarely penetrates beyond 200 meters.

Based on light levels and depth, the ocean is divided into three main zones. The euphotic zone (0–200 meters), also known as the sunlight zone, is where most ocean life thrives since it supports photosynthesis. Below this is the twilight zone (200–1,000 meters), where light is minimal and photosynthesis ceases to exist. From 1,000 meters downward lies the aphotic zone, a region of complete darkness.

The aphotic zone includes the bathypelagic zone (1,000–4,000 m), abyssopelagic zone (4,000–6,000 m), and hadopelagic zone (6,000 m and deeper), each characterized by extreme conditions. But what supports marine life beneath the hadopelagic zone where sunlight is entirely absent?

The Discovery of Dark Oxygen

In July 2024, this question was addressed by Andrew Sweetman of the Scottish Association for Marine Science (SAMS) while studying seafloor ecology and biogeochemistry in the Clarion-Clipperton Zone. This region, located at a depth of approximately 48,000 meters (13,100 feet), is rich in polymetallic nodules containing metals such as nickel, cobalt, and copper.

Despite the complete absence of sunlight, researchers discovered a phenomenon where oxygen is produced in extreme darkness—without sunlight or photosynthesis. This process was named “dark oxygen”, as it is produced chemically rather than biologically.

Electrochemistry in the Deep Ocean

This phenomenon is a remarkable example of chemistry occurring 16,404 feet 2.394 inches deep in the ocean. The Clarion-Clipperton Zone seafloor acts like a natural geobattery, splitting available water molecules through a process known as seawater electrolysis and producing oxygen.

Polymetallic nodules are rock-like formations rich in metals such as cobalt, nickel, manganese, and copper. These metals can behave like miniature batteries, splitting seawater into oxygen and hydrogen in a process similar to electrolysis. When exposed to cold seawater, the nodules release more oxygen than they consume.

Environmental Implications

While oxygen production without sunlight is a groundbreaking scientific discovery, it also provides a crucial explanation for how marine organisms survive in the darkest depths of the ocean. Dark oxygen may have long been sustaining ecosystems in these extreme environments.

However, this discovery raises serious concerns. The growing demand for deep-sea minerals could lead to extensive mining of polymetallic nodules. Such activities may disrupt the natural production of dark oxygen, potentially leading to its permanent depletion and causing devastating consequences for deep-sea marine life.

The discovery of dark oxygen reshapes our understanding of marine ecosystems and their resilience. It reveals the ocean’s hidden ability to sustain life through non-biological processes like seawater electrolysis. At the same time, it emphasizes the urgent need to preserve deep-sea ecosystems to protect one of the ocean’s most vital life-supporting mechanisms.