Dark oxygen
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Dark oxygen production refers to the generation of molecular oxygen (O2) 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.
The theory for dark oxygen production by manganese nodules on the abyssal seafloor is controversial, with scientists disagreeing about its validity.[1][2]
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.[3]
- Oxidation of surface-bound radicals: On silicon-bearing minerals like quartz, surface-bound radicals can undergo oxidation, contributing to O2 production.[4][5][6]
In addition to direct O2 formation, these processes often produce reactive oxygen species (ROS), such as hydroxyl radicals (OH•), superoxide (O2•-), and hydrogen peroxide (H2O2). 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.[7][8]
Biotic production
Biotic production of dark oxygen is performed by microorganisms through distinct microbial processes, including:
- Chlorite dismutation: This involves the dismutation of chlorite (ClO2−) into O2 and chloride ions.[9]
- Nitric oxide dismutation: This involves the dismutation of nitric oxide (NO) into O2 and dinitrogen gas (N2) or nitrous oxide (N2O).[10][11][12]
- Water lysis via methanobactins: Methanobactins can lyse water molecules to produce O2.[13]
These processes enable microbial communities to sustain aerobic metabolism in environments that lack oxygen.
