The discovery that oxygen is produced in the bathypelagic zone means deep sea mining – and the origin of life – needs a serious rethink
Scientists have discovered that oxygen is produced by polymetallic nodules on the deep sea floor, and the aptly named ‘dark oxygen’ is causing scientists to consider how deep-sea mining might affect its supply, and rethink how life on Earth might have originated.
Most oxygen on planet Earth is created through photosynthesis, the process by which plants, algae, and some bacteria use the energy from sunlight to produce chemical energy, creating oxygen as a byproduct. At least 50 per cent of the global oxygen supply comes from the ocean, mostly from phytoplankton.
Sunlight penetrates the upper layer of the ocean – the euphotic, or ‘sunlight’ zone – to a depth of around 200m. A tiny amount light – not visible to the human eye – reaches the dysphotic, or ‘twilight’ zone between 200m and 1,000m, but after that the aphotic, or ‘midnight’ zone remains in perpetual darkness.
Without sunlight there is no photosynthesis and no production of oxygen; those animals that survive in the depths rely on the trace amounts of dissolved gas drawn to the depths by downwellings of cold water. Other deep-sea life survives through chemosynthesis, using symbiotic bacteria to convert toxic chemicals such as the hydrogen sulphide flowing from hydrothermal vents into life-sustaining energy.
A recent study published in Nature Geoscience, however, has found that oxygen appears to be produced by the electrolysis of seawater in areas filled with polymetallic nodules. The potato-sized rocks are the prime target of prospective deep-sea mining operations as a source of rare earth metals such as cobalt, manganese and nickel, which are required to support the increasing demand for consumer electronics and renewable energy technologies.
Led by Professor Andrew Sweetman of the Scottish Association for Marine Science (SAMS), the team behind the discovery were sampling the seafloor of the Clarion-Clipperton zone on behalf of the Metals Company, as part of a study into the potential impacts of deep-sea mining.
The scientists found that oxygen levels over the seabed would rise without any other apparent process affecting the concentration, even going so far as to repeatedly check their sampling equipment for errors when the effect was first noticed.
Under laboratory conditions, Sweetman and his team found that the nodules act like batteries, creating an electrical charge of sufficient power to split seawater into oxygen and hydrogen.
‘If you put a battery into seawater, it starts fizzing,’ said Sweetman. ‘That’s because the electric current is actually splitting seawater into oxygen and hydrogen. We think that’s happening with these nodules in their natural state.’
The Clarion-Clipperton zone, located in the north Pacific Ocean, is of particular importance as it contains dense clusters of polymetallic nodules. This makes it a valuable prospect for the deep-sea mining indiustry, but may also explain why the creation of oxygen is prominent in that particular area.
‘It’s like a battery in a torch,’ said Sweetman. ‘You put one battery in, it doesn’t light up. You put two in and you’ve got enough voltage to light up the torch. So when the nodules are sitting at the seafloor in contact with one another, they’re working in unison – like multiple batteries.’
The discovery also has implications in the quest for answers as to how life on Earth began, as it implies the existence of a factor that has so far not been considered in researching the origins of oxygen-breathing organisms.
‘For aerobic life to begin on the planet, there had to be oxygen and our understanding has been that Earth’s oxygen supply began with photosynthetic organisms,’ said Sweetman. ‘But we now know that there is oxygen produced in the deep sea, where there is no light. I think we therefore need to revisit questions like: where could aerobic life have begun?’
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‘In my opinion, this is one of the most exciting findings in ocean science in recent times,’ said Professor Nicholas Owens, Director of SAMS. ‘The discovery of oxygen production by a non-photosynthetic process requires us to rethink how the evolution of complex life on the planet might have originated.Â
‘The conventional view is that oxygen was first produced around three billion years ago by ancient microbes called cyanobacteria and there was a gradual development of complex life thereafter.Â
‘The potential that there was an alternative source requires us to have a radical rethink.’
The scientists say that more research is needed into how, and how much, dark oxygen is produced before deep-sea mining commences, as there may be an impact beyond the immediate area in which mineral extraction takes place.
‘Through this discovery, we have generated many unanswered questions and I think we have a lot to think about in terms of how we mine these nodules,’ said Sweetman.
‘I don’t see this study as something that will put an end to mining,’ he added, ‘but we need to explore it in greater detail…if we are going to go into the deep ocean and mine it in the most environmentally friendly way possible.’
‘Evidence of dark oxygen production at the abyssal seafloor’ by Andrew K Sweetman et al is published under an Open Access licence in Nature Geoscience.
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