Until now, the deep sea floor was considered to be a zone in which oxygen is consumed – for example through the activity of marine microbes. But now a research team has discovered that oxygen can be produced in the deep sea even in constant darkness. The metal-containing manganese nodules are responsible for this, and are considered a valuable source of raw materials and possible targets for deep-sea mining. It turns out that these nodules act like small geo-batteries: they generate enough electricity to electrolytically split the sea water and thus release oxygen, as the team observed using laboratory measurements. This “dark oxygen production” also sheds new light on the conditions in the primordial ocean and the emergence of the first aerobic life forms, say the researchers.
According to common assumptions, the atmosphere and oceans of the early Earth were still low in oxygen. The first life forms were therefore anaerobic and did not depend on the respiratory gas for their metabolism. Only the spreading cyanobacteria and other photosynthetic single-celled organisms gradually produced enough oxygen to enable higher, aerobic life forms. In the deep sea and other places where there is constant darkness and therefore no photosynthesis is possible, only a small amount of oxygen is produced – most of it is created as a by-product of the anaerobic decomposition of oxygen-containing materials – or so the theory goes.
Oxygen production in permanent darkness
This was also the assumption made by researchers led by Andrew Sweetman from the Scottish Association for Marine Science (SAMS) when they examined samples from the seabed of the Pacific Clarion-Clipperton Zone, the deep-sea area designated as a license area for future deep-sea mining. Manganese nodules are found in large numbers on the seabed, which lies around 4,000 meters deep. These round lumps, which have grown over millions of years, contain valuable metal raw materials such as copper, manganese, nickel, cobalt and rare earths that are important for high-tech applications. In advance of any possible mining of these nodules, scientists from all over the world are intensively investigating which organisms live in the vicinity of these structures and what possible consequences mining would have. For their tests, Sweetman and his team placed sediment and manganese nodules from the Clarion-Clipperton Zone in special deep-sea sample chambers and exposed them to various conditions. They also determined the net oxygen content of the samples.
The evaluation of the sensors showed something surprising: more oxygen was produced in the deep-sea samples than was consumed by the processes taking place. On average, the net oxygen production in the benthic sample chambers was 1.7 to 18 millimoles of oxygen per square meter per day, despite the constant darkness, as the team determined. “When we received this data, we initially thought that the sensors were malfunctioning, because every study carried out in the deep sea to date has only ever found oxygen consumption, not oxygen production,” says Sweetman. But even after recalibrating the sensors several times and taking measurements using an alternative method, the results remained the same. “When both methods produced the same result, we knew we were on to something groundbreaking and previously unimaginable,” Sweetman continues.
Tension-generating tubers
To find out where the deep-sea oxygen comes from, the researchers carried out further tests in the laboratory. The focus was primarily on the manganese nodules, because particularly high oxygen levels had been measured in areas with a high nodule density. “The fact that we also detected oxygen production in ex-situ control samples only with polymetallic nodules also indicated that this was related to their presence,” reports the team. Closer analyses showed that the manganese nodules generate a surprisingly high electrical charge – almost like a battery. “We suspect that this energy comes from the potential differences of the metal ions in the nodule layers,” write Sweetman and his colleagues. This leads to an internal redistribution of electrons and thus generates the electrical voltage. In their tests, the voltage on the surface of some manganese nodules reached up to 0.95 volts. “It seems that we have discovered a natural ‘geobattery’,” says co-author Franz Geiger from Northwestern University in Illinois. “These geobatteries form the basis for a possible explanation for dark oxygen production in the ocean.”
According to this, the electrical voltage generated by the manganese nodules could trigger electrolysis of the seawater surrounding them. As the team explains, around 1.5 volts are enough for this separation of seawater into oxygen and hydrogen. If several manganese nodules lie together on the seabed, their voltage could add up – similar to batteries connected in series – and be enough to start this process. “In my opinion, this is one of the most exciting discoveries in marine research in recent times,” comments Nicholas Owens, Director of the Scottish Association for Marine Science. “The evidence of oxygen production through a non-photosynthetic process forces us to rethink our ideas about the origin of complex life on this planet.” Such “geobatteries” on the seabed could have supplied oxygen for the first aerobic life forms even before the spread of cyanobacteria and other photosynthetic organisms.
At the same time, the discovery of dark oxygen production by the manganese nodules also raises new questions about future deep-sea mining. “This puts a big question mark over the strategies for deep-sea mining, because animal biodiversity is higher in the nodule-rich areas than in the most species-rich tropical rainforests,” says Geiger. The oxygen production that has now been discovered could be one of the reasons for this. But if the nodules are removed, this could damage the deep-sea environment more than previously thought.
Source: Andrew Sweetman (The Scottish Association for Marine Science (SAMS), Oban) et al.; doi: 10.1038/s41561-024-01480-8