Dark oxygen


Dark oxygen production refers to the generation of molecular oxygen 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" 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.
The theory for dark oxygen production by manganese nodules on the abyssal seafloor is controversial, with scientists disagreeing about its validity.

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 O2.
  • Oxidation of surface-bound radicals: On silicon-bearing minerals like quartz, surface-bound radicals can undergo oxidation, contributing to O2 production.
In addition to direct O2 formation, these processes often produce reactive oxygen species, such as hydroxyl radicals, superoxide, and hydrogen peroxide. These ROS can be converted into O2 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:
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 O2 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.File:Cook-Islands nodule 0.jpg|thumb|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.. It has since emerged that manganese nodules were not present in some of the experiments that recorded rising oxygen, and that the result is likely experimental artefact. The authors have since backed away from the two key claims made in the original article - that the rising oxygen levels can be attributed to manganese nodules, and that this is due to the nodules acting like batteries.

    Implications

Despite its diverse pathways, dark oxygen production has traditionally been considered negligible in Earth's systems. Recent evidence suggests that O2 is produced and consumed in dark, apparently anoxic environments on a much larger scale than previously thought, with implications for global biogeochemical cycles.