Dark Oxygen: What this means for the origin of life?

It has long been established that the origin of complex life on this planet began during the Archaeozoic Eon over a billion years ago. When unicellular organisms inhabiting the oceans developed the capacity for oxygenic photosynthesis. This likely evolved in a common ancestor of extant cyanobacteria. The first appearance of free oxygen in the atmosphere is sometimes referred to as the oxygen catastrophe. Evidence of this can be seen in fossil records and geological data from this early stage of Earths lifespan.

However, new research may have thrown this established theory into disrepute. A study by Sweetman, A.K. et al. (Evidence of dark oxygen production at the abyssal seafloor) was published in Nature Geoscience in July 2024 and submits a contrary theory that atmospheric oxygen may have appeared earlier in the timeline than initially thought.

The term “Dark Oxygen” does not refer to the properties or characteristics of the oxygen itself, but refers to the lightless environment in which it is believed to be produced. Typically, oxygen is the by-product of photosynthetic organisms, such as plants, algae and some bacteria. Light energy is converted to chemical energy using carbon dioxide and water to produce glucose and oxygen. For this process to occur successfully, the presence of light is the contributory factor. Light penetration in the ocean decreases rapidly with depth due to water’s ability to absorb light. Sunlight is unable to penetrate below 1000 metres. Therefore, to observe substantial oxygen production in the Abyssalpelagic Zone (4000+ metres) would be inconceivable.

Researchers from a variety of institutions including; The Scottish Association for Marine Science (SAMS), Heriot-Watt University, GEOMAR Helmholtz Centre for Ocean Research, Boston University, University of Minnesota and University of Leeds discovered during a series of in situ benthic chamber experiments; designed to measure respiration rates of deep-sea organisms, an increase in oxygen levels over two days to more than 3 times the background concentration. At depths in excess of 3000 metres, photosynthesis would not be possible. Oxygen is prevalent in deep-sea surface sediments where its rate of consumption reflects the sum of aerobic respiration and oxidation of reduced inorganic compounds produced by anaerobic decay. These processes define Sediment Community O₂ Consumption (SCOC), and quantifying SCOC is needed to estimate fluxes of major elemental cycles through marine systems.

In comparison to previous deep-sea O₂ flux studies that only showed SCOC, the researchers observed that  O₂ consistently accumulated within the chambers and witnessed a net production in O₂, surpassing consumption rates.

The abyssal seafloor at the chamber sites were populated with high concentrations of polymetallic nodules. Samples of these nodules were excavated for the purpose of ex situ testing. External analysis concluded the presence of transition metals, such as Nickel and characteristics offering abundant defect sites, thereby optimising the adsorption of reactants, enhanced conductivity and catalytic performance. The voltage potentials between different positions on the nodules were highly variable, however voltage potentials up to 0.95 V were ascertained.

It was therefore hypothesised that the deep sea nodules may be acting as a “geo battery”, using electrolysis to split seawater into its component hydrogen and oxygen molecules. Not only does this call into question the origin of complex life on our planet but may have significant impact on modern day exploits such as deep sea drilling and mineral deposit harvesting.

The deepest parts of our worlds oceans remain largely unexplored and the discovery of “dark oxygen” demonstrates just how much is still unknown about these extreme depths. Environmental activists have campaigned for decades to prevent deep sea mining of the Pacific Ocean due to its fragile ecosystem. This latest discovery only exacerbating the damage that unchecked industrialisation could have on the planet. The vast underwater region of the Pacific Ocean between Mexico and Hawaii is of growing interest to mining companies specifically due to the presence of these polymetallic nodules. Rich mineral reserves of manganese, nickel and cobalt constitute the majority of these nodules and are highly sort after components for the development of electric car batteries and low-carbon technologies.

These latest findings have not been without their share of controversy. Some members of the scientific community have reserved judgement regarding this dramatic new theory. Some of whom have refuted the discovery altogether. This is not uncommon with emerging and divisive scientific principals. The research team demonstrated a series of alternative explanations to their findings but remained conclusive that a form of electrolysis was the likely precursor to their abnormal oxygen readings.