RESOURCES:
Nature Chemistry: "Secondary-sphere hydrogen bonding promotes catalytic nitrate reduction at iron"
TRANSCRIPTION:
Caroline MacGregor: This is 89.1 WEMU. And today, we are going to be talking about nitrates and their role in nature and specifically with regard to supporting plant growth but also how they can damage ecosystems when there is excessive algae growth. To discuss all of this, I am joined by U-M Professor of Chemistry Nathaniel Szymczak. Professor Szymczak led a recent study highlighted in the publication "Nature Chemistry," which is supported by the National Institutes of Health and the U.S. National Science Foundation. Welcome, Professor Szymczak!
Dr. Nathaniel Szymczak: Thanks! Great to be here!
Caroline MacGregor: For the sake of our listeners, or the average layperson, tell us about this study and what prompted it. I'm quite curious to know about what you discovered as far as preventing nitrate pollution.
Dr. Nathaniel Szymczak: Yeah. Thanks for the question! You know, at the essence, it really comes down to fertilizers. So to maximize crop yields, farmers apply lots and lots of nitrogen-based fertilizers, really just to generate enough plants and food so that we can sustain the population. So this extra nitrogen that's applied ultimately forms nitrate. And when it rains after applying fertilizer, the nitrate that can't be taken up by plants ultimately gets washed into water bodies. And nitrate causes algal blooms, and it actually changes the aquatic ecosystem so that things like plants and aquatic life can't survive. And so this is the problem. Nitrate is a pretty unreactive molecule, and it's tough to remediate. And ultimately, what we found is that human intervention has actually changed the amount of nitrate into the ecosystem, such that the systems that we normally have in place in biology just physically can't keep up. This is analogous to maybe the carbon dioxide problem, where plants similarly can take up carbon dioxide. But with human intervention, we're putting so much extra molecules, either nitrate or carbon dioxide, into the environment that the biosphere simply can't keep up. So our goal was actually to tackle the problem and figure out a new catalytic system that converts nitrate into a value-added product or basically capture it from the environment and then convert it to something else.
Caroline MacGregor: And as I understand it, just quickly, nitrates are produced naturally when microorganisms break down nitrogen-containing organic matter. Is this correct?
Dr. Nathaniel Szymczak: That's right. So it turns out that nitrate is what we call the "terminal chemical product" of all nitrogen that is applied maybe to crops, so manure, all fertilizer. If you give it enough time in the environment, it always forms to just one molecule, and that's nitrate.
Caroline MacGregor: And back to the problem that they can cause when there's an excess of them, let's delve into your study a little bit more and what your team determined as far as keeping nitrates in check.
Dr. Nathaniel Szymczak: Right. So, it turns out there's actually not a lot of chemical design principles to make what we call catalysts to convert nitrates into other molecules. And so we look to biology, we look at nature, of how this type of transformation is accomplished. And there are these enzymes--proteins--called nitrate transporter proteins and nitrate reductase enzymes. And in biology, these are the enzymes that are responsible for grabbing onto nitrate and converting that back into an environmentally benign molecule, dinitrogen. And nitrogen is about 80% of the air that we're breathing right now. And I'm a chemist, and we looked at the kind of molecular level details of how these enzymes work. And we find that the way in which biology achieves this is through a network of so-called hydrogen bonds within their active site. And these are things like kind of think about a molecular spiderweb that's cast around the outside periphery of a very larger structure, and it serves to grab on to capture nitrate molecules. And these molecular spiderwebs that I mentioned, these are the same very weak interactions called hydrogen bonds that pull together your DNA and actually cause water to be a liquid rather than a gas.
Caroline MacGregor: I love that term. "Molecular spiderwebs." It sounds like something out of a science fiction movie.
Dr. Nathaniel Szymczak: That's it, yeah! It's true! I think it's kind of a good way to think about what these types of interactions are. They're kind of like casting a net, almost like a fishing net or a spiderweb. And they grab onto the nitrate molecule. And one of the interesting things about our study is that we found that these well-positioned hydrogen bonds that we've prepared in our synthetic systems, they not only grab onto nitrate molecules, but they physically redistribute charge within that molecule and make it much easier to convert to other types of molecules--value-added molecules. And that was one of the very exciting aspects. And once we developed that system, we found that we can apply light and convert nitrate all the way back to ammonia, which is a very value-added product.
Caroline MacGregor: If you're just joining us, my guest today is Nathaniel Szymczak. He is the corresponding author of a study that has delved into nitrates and their role in nature. This study was highlighted recently in the publication "Nature Chemistry" and is supported by the National Institutes of Health and the U.S. National Science Foundation.
Dr. Nathaniel Szymczak: So we took the inspiration of the biological systems, and we designed a synthetic system where we position these secondary interactions, these web-type motifs, around an active site metal. And what we found is that collectively the metal and the web, so to speak, captures nitrate with really high affinity, but it also does something very special. Once nitrate is captured in this web, it basically redistributes the charge, and it enables nitrate to be converted into other molecules like ammonia. And this is a molecule that is used on a very, very large scale. When you look at the molecular environment of many things in nature, things look like a science fiction movie. And it's exactly that. So this halo is the same as this molecular spiderweb that I mentioned. And we found that we can add nitrate and light with a chemical reductant and convert nitrate all the way back to ammonia at very, very high efficiencies. And ammonia is a molecule that is generated on an extremely large scale industrially. So, we use, right now, if you think about the global energy, so if you think about all of the energy that's used in the entire world, we use about up to 2% of global energy just to do one chemical reaction, and that's to convert nitrogen into ammonia. And ammonia is a form of what we call fixed nitrogen. It's biologically available nitrogen. And the fact that we use so much energy to make ammonia really underlies the problem of the requirement of feeding the planet. And so our process provides a single-step process to convert nitrate all the way back to ammonia in one step. So we're taking the most, what we call, oxidized form of nitrogen all the back to the most reduced form of nitrogen, so that it can be maybe recycled in a different way.
Caroline MacGregor: How long did your study take? And how many people worked on your team? And with regards to the implications of the study, tell us also about your findings, as far as using nitrates in a positive way for agriculture and the planet in general.
Dr. Nathaniel Szymczak: Right. So, the reality of fundamental chemical science that's done at universities is that things take a long time. So this study took three or four years from its start to its completion, and it took up to five people, up to five scientists that have contributed in various aspects. And basically, the key outcome of this study is basically a new set of design principles. So, as I mentioned earlier, we don't have a lot of great ways if we're trying to make a catalyst that can process nitrate into other types of molecules. We don't have a great design principle of how to do that. So, our study provides a new way of how to do it. It kind of teaches the community some chemical principles that might underlie subsequent remediation strategies. So I would say we're not close yet to converting these types of results into an industrial process, but the goal is that potentially other people can use these insights in new catalyst development.
Caroline MacGregor: All right. And the publication "Nature," they talked about the tendency to over-fertilize crops, and a lot of the fertilizer, when it's applied, leeches as runoff into streams and groundwater and lakes. And of course, here in Washtenaw County, everybody's very concerned with the environment and making sure that we protect our natural habitat as best we can. What implications does all of this have for us here?
Dr. Nathaniel Szymczak: The challenge is that there's a balance. So when we grow crops, we want to maximize the amount of food that we can generate, and the way in which we do that is to apply fertilizers. So if we don't apply any fertilizers, we'll generate less food. So what farmers do is try to maximize the amount of food output by maximizing the amount of fertilization. And the challenge is, as you mentioned, a lot of that fertilizer just physically isn't taken up by the plant, and it's ultimately converted into runoff, which causes eutrophication, things like that, in our waterways. So it's a little bit of a balancing act, in terms of prioritizing either food or environmental problems. That's kind of a challenge.
Caroline MacGregor: So how do you foresee your study being used in a positive way, as far as agriculture in the future and how we can stop some of this pollution?
Dr. Nathaniel Szymczak: You know, I think this provides a step forward to understanding maybe another remediation strategy. Our study has kind of outlined maybe a key set of design principles that are needed for subsequent remediation. And these might happen five, 10, 20 years down the road, but it's a necessary step en route to future remediation.
Caroline MacGregor: I've been talking with Professor Nathaniel Szymczak, and we've been discussing nitrates and how they can damage ecosystems when there is excessive algae growth. Professor Szymczak, thank you so much for joining us today!
Dr. Nathaniel Szymczak: Thanks, Caroline! I enjoyed chatting with you!
Caroline MacGregor: This is 89.1 WEMU, Ypsilanti.
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