# Dark oxygen

Dark oxygen production refers to the generation of molecular oxygen (O$_2$) through processes that do not involve light-dependent oxygenic photosynthesis. The name therefore uses a different sense of 'dark' than that used in the phrase "biological dark matter" (for example) which indicates obscurity to scientific assessment rather than the photometric meaning. While the majority of Earth's oxygen is produced by plants and photosynthetically active microorganisms via photosynthesis, dark oxygen production occurs via a variety of abiotic and biotic processes and may support aerobic metabolism in dark, anoxic environments.

## Abiotic production

Abiotic production of dark oxygen 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$_{2.}$
* Oxidation of surface-bound radicals: On silicon-bearing minerals like quartz, surface-bound radicals can undergo oxidation, contributing to O$_2$ production.
In addition to direct O$_2$ formation, these processes often produce reactive oxygen species (ROS), such as hydroxyl radicals (OH$^•$), superoxide (O$_2$$^{•-}$), and hydrogen peroxide (H$_2$O$_2$). These ROS can be converted into O$_2$ and water either biotically, through enzymes like superoxide dismutase and catalase, or abiotically, via reactions with ferrous iron and other reduced metals.

## Biotic production

Biotic production of dark oxygen is performed by microorganisms through distinct microbial processes, including:
* Chlorite dismutation: This involves the dismutation of chlorite (ClO$_2$$^−$) into O$_2$ and chloride ions.
* Nitric oxide dismutation: This involves the dismutation of nitric oxide (NO) into O$_2$ and dinitrogen gas (N$_2$) or nitrous oxide (N$_2$O).
* Water lysis via methanobactins: Methanobactins can lyse water molecules to produce O$_{2.}$
These processes enable microbial communities to sustain aerobic metabolism in environments that lack oxygen.

## Experimental evidence

Recent studies have provided evidence for dark oxygen production 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 O$_2$ 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.A bed of manganese nodules offshore of the Cook Islands
* Seafloor environments: A study on manganese nodules on the abyssal seafloor has suggested abiotic dark oxygen production. The proposed mechanism is electrolysis, because voltages were recorded on the surface of the nodules. However, no voltage great enough to split water was measured, the energy source for electrolysis is unknown, and previous experiments from the same region have not found any evidence of oxygen production.

## Implications

Despite its diverse pathways, dark oxygen production has traditionally been considered negligible in Earth's systems. Recent evidence suggests that O$_2$ is produced and consumed in dark, apparently anoxic environments on a much larger scale than previously thought, with implications for global biogeochemical cycles.