Deep-Sea ‘Nodules’ May Produce Oxygen, Study Finds

CHARLES BERGQUIST: This is Science Friday. I’m Charles Bergquist, sitting in for Ira Flatow.

Later in the hour, why COVID isn’t taking a summer vacation. Plus, a tale of high-flying turkey vultures. But first, the deepest parts of the ocean are home to some of the most incredible creatures on our planet. Though it’s dark and cold, you’ve got things like anglerfish, bioluminescent jellyfish. It can feel like a completely different world. And the creatures down there don’t just look different. They’ve developed unusual ways to survive the harsh conditions of the super deep.

An international team of researchers recently discovered that some 13,000 feet below the surface, oxygen can be produced in a way that they weren’t expecting– without photosynthetic life. Small lumps, called polymetallic nodules, at the bottom of the ocean were able to produce enough electricity to break down water and make oxygen. That challenges the idea that photosynthesis is necessary to produce much oxygen. But while it gives some answers as to how life can thrive at the bottom of the sea, it also raises a whole load of new questions.

Here to answer some of them is the lead electrochemist of that research team, Dr. Franz Geiger. He’s the Charles E. and Emma H. Morrison, Professor of Chemistry at Northwestern.

Welcome to Science Friday, Dr. Geiger.

FRANZ GEIGER: Thank you so much, and thank you for covering this exciting story.

CHARLES BERGQUIST: Oh, it’s our pleasure. It’s very cool. Tell me a little bit about how the team came to this discovery.

FRANZ GEIGER: This goes back about 10 years, when the lead author on this work, Professor Andrew Sweetman, of the University of Highlands and Islands, in Scotland, had a interesting discovery that he didn’t believe, which was that when they analyzed oxygen readings down below at the abyssal seafloor, the oxygen increased over time. And that shouldn’t have happened because there was no light to produce photosynthesis. It’s completely dark down there. And the surprise was such that the team decided this can’t be right. And they didn’t believe their own data for a long time, and kept going on tours out in the ocean, repeating measurements with different instrumentation, and getting the same increases in oxygen as they did in the first set of measurements.

And so Andrew contacted me about a year ago, and had read a paper of ours that we published on electricity generation on metallic nanolayers. And I said, I don’t think that’s enough power that can lead to the electrolysis of water, or water splitting. You need about 1.5 volts or 1 volt for that– for seawater– and our earlier paper had shown only 10 millivolts, 100 times– or 10 times smaller voltages. But I said, why don’t you send some samples? And the rest is history.

CHARLES BERGQUIST: So what are these polymetallic nodules? Would I know one if you put one on my desk? What does it look like?

FRANZ GEIGER: They are fascinating structures. They look like truffle– large truffles– for which you would pay a king’s ransom. But they don’t have the nice aroma. They are not as dense as a big chunk of metal– let’s say iron– of the same size, but much lighter. Many people liken them to a potato. They have about that size, on average. And when you go down there and take pictures with the camera that’s got a light attached to it– because, again, it’s completely dark otherwise– you see hundreds and hundreds of miles of these nodules on the seafloor. They look like a sack of potatoes thrown on the floor.

CHARLES BERGQUIST: Yeah. Where do they come from? How does one of these nodules form in the first place?

FRANZ GEIGER: They come from the ocean. Believe it or not, we’ve got many metals in ionic form that are dissolved in seawater. And that goes back hundreds of millions of years. And under the conditions of high pressure and low temperature that are met down there at 3 to 4 kilometers depth, over the years, these ions precipitate out onto sharp objects. Often, it’s literally a shark tooth that has fallen to the ground from high above. And many times people, in fact, find those nucleation sites– shark teeth from 10, 20, 50 million years ago– at the center of one of these nodules.

And they grow at about 1 millimeter per 1 million years. That’s very slow. And given the size and isotope measurements, et cetera, that can determine their age, they are about 100 million years old. The one I have in my lab, and that we did measurements on that set of samples, is about 100 million years old. It’s the oldest thing I’ve ever worked with in my lab. It’s absolutely fascinating.

CHARLES BERGQUIST: Wow. So you’ve got deep seawater. You’ve got your 100-million-year-old potato. How does that work into oxygen production? What’s the actual process going on here?

FRANZ GEIGER: We hypothesized that it’s because of electrochemistry. And the reason is that, when Andrew had sent his nodules to us, we used this high-sensitivity voltmeter that we used for our previous study and attached it to platinum electrodes that we then attached to the nodules, sitting inside a beaker that simply had ocean water simulant in it. And the readings were off the charts.

I had expected a few millivolts at most. We read 50 millivolts, 150 millivolts, 950 millivolts. So close to what you get out of a AA battery, which is the classic kindergarten or high school experiment that people do in a science class, where you take your battery, you hook it up with wires, and stick the wires into saltwater. And on one side you get hydrogen bubbles coming out and on the other wire you get oxygen bubbles coming out.

Now, our sensors were only sensitive to oxygen. They were not sensitive to hydrogen. So we have evidence for oxygen production through those sensors– meaning evidence for what is called the oxygen evolution reaction. That’s one half of water splitting. And how that exactly occurs within the nodules, we hypothesize, is because of the layered structure of the nodules. When you cut one open, it almost looks like an onion, or perhaps a tree. So it’s got a ring structure, where there’s more cobalt deposited for the first 10 million years, then manganese the next 10 million years, a little bit more iron the next 10 million years.

And so every time you’ve got these gradients of metal concentrations, you effectively have a battery that’s been known since Alessandro Volta, when he built the voltaic pile over 200 years ago.

CHARLES BERGQUIST: Wow. So these nodules are acting like their own batteries. If I did my kindergarten-level science experiment and hooked one up, would I actually see bubbles coming off of these potatoes? How much oxygen are they producing?

FRANZ GEIGER: The bubbles are seen when you take the nodules and water from the ocean floor– again, at these very, very high pressures– to the surface. Professor Sweetman has done that measurement. And you can see, once you vent that benthic chamber– that instrument that collected the nodules with the water– it looks like a glass of sparkling water.

Down at the ocean’s floor, the pressures are so enormous and the temperature is so low that oxygen does not bubble out of these nodules. Oxygen dissolves into the seawater. The solubility goes up with pressure. And just like sugar dissolves in water, the oxygen will dissolve in the water. So no bubbles are seen. But you can still measure them using the oxygen sensors that we have.

CHARLES BERGQUIST: So what does this discovery mean in terms of chemistry and deep-ocean research? How does it challenge preconceived notions here?

FRANZ GEIGER: It does so in a number of ways. And we are in the process of putting grant proposals together for a variety of institutions because this is obviously opening up lots of questions, as you indicated in your introduction. The nice thing is that the picture now is less murky than what it was before because we have this new information. But we don’t know, for example, the rates at which oxygen is produced. So we don’t know how it competes with other oxygenation pathways such as photosynthesis.

We also don’t know whether the nodules are always active through their entire lifetime or if they have active periods that may have lasted for a million years 50 million years ago, but are now inactive, and other ones that are currently active only came to life, so to say, maybe a million years ago.

The other thing we don’t know is whether or not that oxygen could act as a source for living organisms at this very diverse area of the ocean, which is the abyssal seafloor.

CHARLES BERGQUIST: Should I be thinking of these nodules as something like a battery that is being consumed in the course of this electrochemical reaction, or is it a catalyst that just facilitates the reaction?

FRANZ GEIGER: That’s an excellent question and one of the central questions we have here in one of the proposals that we’re currently putting together. And we don’t know the answer yet. It’s likely that these structures are active for some time and then deactivate, which means that, if they’re catalytic in nature, that extent of catalytic action is time dependent. It could also be that it’s just simply acting as a battery. And at that point, just like any battery will empty out at some point, some of these nodules– or many of them– will probably not be active today.

CHARLES BERGQUIST: So I know that there are companies that have proposed doing deep sea mining, essentially, to gather these nodules. Do you think that that’s an environmental risk if these nodules might be providing valuable oxygen to the deep-sea environments?

FRANZ GEIGER: We hope that this study will inform on when, where, and how to mine, if mining licenses are given out. It’s important to state that no mining licenses have been issued, but only exploratory licenses. This is a classic dilemma. We desperately need the minerals for preparing the energy transitions out of a fossil fuel economy into one that’s powered by batteries. And at the same time, if we open up the seafloor for mining, again, we hope that the study informs on how to do that with the least environmental and biological and ecological impact.

CHARLES BERGQUIST: So if this is a previously unknown way of producing oxygen on earth, might this tell us anything about other planets? Like, if you see free oxygen in a planet’s atmosphere, it doesn’t necessarily have to have come from life.

FRANZ GEIGER: Correct. So we are very excited about the possibility of having ocean-bearing moons that have been hypothesized to exist, with lots of evidence for it, but that are coated by thick shields of ice. Of course, those oceans are entirely dark. So it could be that we now have a double-dark scenario, where there’s dark oxygen being produced in dark oceans on moons that perhaps could serve as an oxygenation form of life on those planets or moons.

CHARLES BERGQUIST: So what do you want to learn next about this? What’s the next step here?

FRANZ GEIGER: I’m very excited about seeing whether or not there might be a blueprint down there at the bottom of the ocean in these nodules to help us build better catalysts here at the Earth’s surface. Many of us in the sciences are working on better water-splitting catalysts. The best ones are platinum group elements. They’re very rare. So that’s not going to be promising for the energy transition.

The metals that are electrochemically active in the nodules are common– nickel, iron, manganese, cobalt. And we think that there might be a plan that we can pursue to build such structures up here.

CHARLES BERGQUIST: Dr. Franz Geiger is the Charles E. and Emma H. Morrison Professor of Chemistry at Northwestern University. Thanks so much for taking the time to talk with me today.

FRANZ GEIGER: Thank you so much. I really appreciate being on Science Friday.