Metallic nodules found deep in the sea seem to be producing significant amounts of oxygen by some unknown mechanism, researchers revealed last year. Now, we may have found out how it is happening 鈥� and the same process could also produce oxygen to help terraform Mars.
at the Chinese Academy of Sciences in Beijing and his colleagues have discovered that two species of deep-sea bacteria can produce large quantities of oxygen. What鈥檚 more, this process might form the metallic nodules as a side effect, explaining why these minerals seem to produce this 鈥渄ark鈥� oxygen.
鈥淚 am very surprised by the amount of oxygen production by these deep-sea bacteria,鈥� says Sun. 鈥淭his pathway operates independently of solar radiation and photosynthesis.鈥�
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The scientific orthodoxy is that all the oxygen in the oceans comes from photosynthesis 鈥� the light-driven splitting of water by organisms in the upper waters into oxygen and hydrogen. In recent years, it has been discovered that a few bacteria and archaea can produce oxygen in the dark, but the quantities are so small they weren鈥檛 thought important.
Then in 2013, at the Scottish Association for Marine Science detected oxygen production on the abyssal plain thousands of metres down. He dismissed it as an instrument error, but when he found the same thing again years later, he began investigating.
Last year, his team published a controversial paper claiming that metallic nodules on the seabed produce enough oxygen to shape local ecosystems and might have even played a role in the evolution of life. But critics raised various objections, including pointing out that the nodules contain manganese oxide that can form only if oxygen is already present 鈥� probably ruling this process out as relating to the first life.
Now Sun鈥檚 team has shown that when two strains of deep-sea bacteria are grown in the presence of nitrate (NO3 ions), they reduce it to ammonium, releasing oxygen in the process 鈥� a previously unknown reaction. 鈥淭he dissolved oxygen concentration is over 300-fold higher than that produced by a previously reported ammonia-oxidizing archaea,鈥� says Sun.
Such oxygen could be crucial for some oxygen-dependent microbes and even small animals, the team suggests, both on Earth and perhaps elsewhere, too. 鈥淢icrobial dark oxygen could be a significant oxygen source in nitrate-rich ocean worlds, even in the absence of light,鈥� the team鈥檚 paper states. Nitrates have been found on Mars, so this process could help terraform the planet, the team suggests. 鈥淭he concentrations are sufficient to support the microbial nitrate-dependent anaerobic oxygen production we propose,鈥� says Sun.
What鈥檚 more, Sun鈥檚 team also found that when manganese is present, manganese oxide precipitates out, suggesting the metallic nodules could be formed by bacteria.
Sweetman says the findings could explain the oxygen production his team reported. 鈥淚t鈥檚 all very exciting,鈥� he says. Nitrates are plentiful in the sediments on the seafloor, says Sweetman. 鈥淪o you have quite a lot of stuff that can be used for this process.鈥�
His team did try to rule out a microbial explanation by adding a poison to kill off any microbes, leading them to instead suggest an electrochemical process might be involved. 鈥淸But] we could not guarantee that the poison seeped into all of the available pore-spaces in the sediment and nodules,鈥� says Sweetman.
Other explanations for the oxygen production remain possible, says Sweetman. 鈥淢aybe there鈥檚 some connection here. Maybe there isn鈥檛,鈥� he says. 鈥淲e are still in the process of trying to figure out what is causing it.鈥�
鈥淭he thing that鈥檚 key here, I think, is that almost every couple of months we鈥檙e discovering a new process that microbes carry out, which we didn鈥檛 think possible,鈥� he says.
But at the University of Southern Denmark isn鈥檛 convinced by Sun鈥檚 findings. 鈥淚f true, the results would be outstanding, but I have many concerns,鈥� he says, adding that he is sceptical that a process like this is a significant source of oxygen in the deep ocean. 鈥淒eep ocean oxygenation is due to the patterns of ocean circulation, and this is well known.鈥�
鈥淲e鈥檒l see where we are in the next five years,鈥� says Sweetman. 鈥淚 think this dark oxygen production is widespread. We鈥檙e just starting to scratch the surface here.鈥�
One reason why his claim of oxygen production involving the metallic nodules is so controversial is that companies are interested in mining such minerals. The findings suggest deep-sea extraction of nodules would have a bigger impact than currently thought. If the nodules and top 10 centimetres of sediment were removed, it would take 100,000 years for the ecosystem to return to the same state, says Sweetman.
鈥淚鈥檓 not for mining, but I鈥檓 not against it,鈥� he says. 鈥淏ut I think you need to be mindful of just how long these systems will take to recover.鈥�
bioRxiv
