Research
Background
Polymetallic nodules that abound in parts of the deep Pacific Ocean contain rare and valuable elements that could enable the transition from fossil fuel to more renewable energy sources. But the process of collecting these nodules is likely to have profound impacts on deep-sea ecosystems. Researching the impacts of such deep-sea mining, an international team of researchers made an unexpected discovery that they initially dismissed as a measurement error: they found that oxygen can be produced in the darkness of the deep sea.
Oxygen is one of the six most important elements for life on Earth: a fundamental building block for biomolecules and essential for aerobic respiration. Before this find scientists believed that all free oxygen in air and water had for the past 2.7 billion years been produced as a by-product of oxygenic photosynthesis by autotrophic organisms – initially bacteria, then algae and plants. As photosynthesis requires light, all oxygen in the deep sea was thought to originate in lit surface waters and to reach the deep when surface water sinks, mixes and circulates.
When the research team found that oxygen concentrations rose in the water surrounding polymetallic nodules in the Clarion Clipperton Zone in the Pacific, they explored different potential causes and found that the voltage difference across the nodules was so high, that it could split water molecules into hydrogen and oxygen.
Question 1
How widespread is dark oxygen production in abyssal plain environments?
Polymetallic nodules that abound in parts of the deep Pacific Ocean contain rare and valuable elements that could enable the transition from fossil fuel to more renewable energy sources. But the process of collecting these nodules is likely to have profound impacts on deep-sea ecosystems. Researching the impacts of such deep-sea mining, an international team of researchers made an unexpected discovery that they initially dismissed as a measurement error: they found that oxygen can be produced in the darkness of the deep sea.
Oxygen is one of the six most important elements for life on Earth: a fundamental building block for biomolecules and essential for aerobic respiration. Before this find scientists believed that all free oxygen in air and water had for the past 2.7 billion years been produced as a by-product of oxygenic photosynthesis by autotrophic organisms – initially bacteria, then algae and plants. As photosynthesis requires light, all oxygen in the deep sea was thought to originate in lit surface waters and to reach the deep when surface water sinks, mixes and circulates.
When the research team found that oxygen concentrations rose in the water surrounding polymetallic nodules in the Clarion Clipperton Zone in the Pacific, they explored different potential causes and found that the voltage difference across the nodules was so high, that it could split water molecules into hydrogen and oxygen.
Question 2
Is hydrogen released during electrolysis-driven DOP and does this play a
role in deep-sea food webs?
If electrolysis of water releases hydrogen into the environment this could stimulate the chemosynthesis of new biomass and add a new food-input pathway in food-poor deep-sea habitats (figure 1). If this occurs, this might reduce the impact of climate-change-induced changes in the deep sea, as the system would not solely depend on reduced surface ocean productivity and carbon flux. Sweetman et al (2019) indeed found that chemosynthetic rates were >50 times higher at polymetallic nodule-covered seafloor sites than could be explained by ammonium fluxes. This could be linked to hydrogen production in nodule areas.
To answer this question we will track chemosynthetic activity in situ using isotope labelling experiments with 13C labelled bicarbonate.
We will also have hydrogen sensors inside the benthic chambers to study hydrogen evolution throughout the experiments.
We will study the pressure dependence of the overpotentials associated with the oxygen evolution reaction, map the speciation of polymetallic nodules at the macro- and nanoscale as well as electrochemical pathways on and within nodules, search for covariance of high voltage differences in areas of high variation in chemical speciation and of high voltage differences with internal surface area, and seek to establish a causal relation between macro- and nanoscale speciation and electrochemical activity.
Question 3
How could DOP be affected by deep-sea mining activities, such as sediment plume sedimentation onto metal-oxides?
If we find that DOP is produced via electrochemical or microbial processes, deep-sea mining might significantly impact this process as it could kill the microbial community during the disturbance and/or bury the electrochemically reactive surfaces of the nodules.
We will test whether DOP is reduced when sediment smothers nodules by injecting slurry into seabed chambers and monitoring DOP activity.