Dark oxygen production is the formation of molecular oxygen by pathways independent of light-dependent oxygenic photosynthesis. Although plants and photosynthetically active microbes produce the majority of the oxygen on Earth through photosynthesis, DOP occurs due to abiotic and biotic processes and may sustain aerobic metabolism in the dark, anoxic settings.

Abiotic DOP

Abiotic DOP can occur through many pathways, such as the following:

Radiolysis of water

This process normally takes place in dark geological environments, such as aquifers. because here decaying radioactive elements in surrounding rock cause the decomposition of water molecules to make O2.

Oxidation of the surface-bound radicals

The surface-bound radicals may get oxidized on the silicon-bearing minerals such as quartz to produce O2.

In addition to the direct production of O2, these reactions generally also produce ROS, such as hydroxyl radicals (OH•), superoxide (O2•-), and hydrogen peroxide (H2O2). These ROS can then be biotically converted to O2 and water by enzymes including superoxide dismutase and catalase or abiotically by reaction with ferrous iron.

Biotic DOP

It is the biotic DOP, accomplished by microorganisms with the help of distinct microbial processes such as:

Chlorite dismutation

It is a process of the dismutation of chlorite (ClO2-) into O2 and chloride ions.

Nitric oxide dismutation

consists of the dismutation of nitric oxide (NO) to form O2 and nitrogen gas (N2) or nitrous oxide (N2O).

Water lysis by methanobactin

The methanobactin have the capability to lyse water molecules to produce O2. These processes enable microbial communities to maintain aerobic metabolism in anoxic environments.

Experimental evidence

Recent evidence has indeed pointed through various remarkable case studies to the compelling existence of DOP in various geological and subsurface environments

Groundwater ecosystems

Until recently, dissolved oxygen concentrations had been measured in ancient groundwaters that had either to been presumed to be anoxic. The presence of O2 is attributed to microbial communities capable of producing dark oxygen and water radiolysis. This local generation of oxygen is also supported by metagenomic analyses and oxygen isotope studies rather than atmospheric mixing.

A bed of manganese nodules offshore of the Cook Islands Deep-sea environments

Proof for this has further been suggested to take place deep inside the sea’s floor; this is particularly the case for those that have a high concentration of polymetallic nodules. The nodules have been said to be able to produce sufficient electrical current so as to allow seawater electrolysis and the resultant production of O2.

Consequences

Although the different pathways involved, DOP has traditionally been considered negligible in Earth systems. Recent evidence shows that O2 is being produced and consumed in the dark in an apparently anoxic environment at a larger scale than previously considered; this extends to global biogeochemical cycles.