Iron oxide acts as a natural catalyst for unlocking phosphorus, which is fueled by plant growth

Researchers at Northwestern University are actively overturning the traditional view of iron oxide as merely phosphorus. A lifelong important nutrient, most phosphorus in the soil, is organic matter from plants, microorganisms, or animal debris. However, plants need inorganic phosphorus, a type found in fertilizers, for food.

Researchers traditionally thought that only enzymes from microorganisms and plants could convert organophosphorus into inorganic forms, but Northwest scientists previously discovered iron oxides in natural soils and sediments that could previously facilitate the conversion.

Now, in a new study, the same research team has discovered that it does not produce negligible amounts of valuable resources for iron oxide. In fact, iron oxide is a very efficient catalyst and can drive conversions at speeds comparable to enzyme reactions. This finding could help researchers and industry experts to better understand the phosphorus cycle and optimize its use, especially in agricultural soils.

This study was published today in the Journal of Environmental Science and Technology.

“Linn is essential for all forms of life,” said Ludmira Aristilde of Northwestern, who led the study. “The backbone of DNA contains phosphates. Therefore, all life on Earth, including humans, depends on phosphorus to flourish. Therefore, fertilizer is necessary to increase the phosphorus of the soil. Otherwise, the crops needed to feed the planet will not grow.

An expert on organic matter dynamics in environmental processes, Aristilde is an associate professor of environmental engineering at the McCormick School of Engineering in Northwestern. She is also a member of the Center for Synthetic Biology, the International Institute for Nanotechnology, and the Paula M. Trienel Institute for Sustainability and Energy. Jade Basinski, PhD student at Aristilde’s Laboratory, is the first author of the paper. Students and postdoctoral researchers from the other PhD Aristilde team contributed to the work.

The road to access Lin

For centuries, farmers have added phosphorus to their fields to improve crop yields. Not only does it improve crop quality, but phosphorus also promotes root and seed formation. Plants literally cannot survive without it.

But there’s a catch. Plants have evolved to use phosphorus in the simplest and most readily available form: inorganic phosphorus. Inorganic phosphorus is like a ready-to-use molecule that plants can easily consume and incorporate into their metabolism.

However, most phosphorus in the environment is organic and means that it is attached to a carbon atom. To access this phosphorus, plants rely on their own secretory enzymes or enzymes secreted by microorganisms to break the binding of organophosphorus and release the inorganic form that can be used.

In previous research, Aristilde’s team discovered that it is not just a vehicle capable of performing this essential transformation. Iron oxide, which naturally occurs in soil and sediments, can also carry out reactions that convert organophosphorus to produce inorganic forms.

How much and how fast?

After proving that iron oxides provide another pathway for plants to access phosphorus, Aristildo and her team sought to understand the speed and efficiency of this catalytic conversion.

“Iron oxides trap phosphorus because they have different charges,” Aristilde said. “Iron oxide is actively charged and phosphorus is negatively charged. This means that phosphorus is linked to iron oxide everywhere. Previous studies have shown that iron oxide serves as a catalyst for cleaving phosphorus.

To explore this question, researchers investigated three common iron oxides: umata, hematite and ferrihydrite. Using advanced analytical techniques, Aristilde and her team studied the interactions between these iron oxides and the various structures of ribonucleotides, which are RNA and DNA components.

In their multiple experiments, Aristilde’s team searched for inorganic phosphorus both in the surrounding solution and on the surface of iron oxide. By running the experiments over a specific period and running the experiments with different concentrations of ribonucleotides, the team determined the reaction rate and efficiency.

“We concluded that iron oxide is a ‘catalyzed trap’ because it catalyzes the reaction to remove phosphoric acid from organic compounds and traps phosphate products on the surface of minerals,” Aristilde said. “The enzymes don’t lock the product. They make everything available. We found that getite is the only mineral that didn’t catch all phosphorus after the reaction.”

The team found that each type of iron oxide exhibited varying degrees of catalytic activity to cleave phosphorus from ribonucleotides. Goethite was more efficient with ribonucleotides containing three phosphorus, whereas hematite was more efficient with ribonucleotides containing one phosphorus. Hematite is found in the Midwest of the United States, whereas getite is commonly found in soils in the Southern United States and South American countries.

What’s next?

Next, the team at Aristilde tries to understand why different iron oxides have different efficiencies for catalytic processes and how Goethite releases phosphates, but ferrihydrite and hematite trap all the phosphates produced. Researchers initially assumed that the surface structure of phosphorus compounds plays a role, but they could not find a clear trend. Now they believe that the chemistry of the mineral itself may be the secret behind its success.

Phosphorus is a finite resource and therefore its supply is declining as it is mixed from phosphate rocks found only in the US, Morocco and China. Farmers and researchers are concerned that phosphorus will ultimately become very expensive, which will increase the overall food cost and make basic staples affordable.

Therefore, finding new ways to convert trapped organophosphorus into bioavailable inorganic phosphorus is essential to global food supply.

“Our work provides a stepping stone to design and engineer synthetic catalysts as a way to recycle phosphorus,” Aristilde said. “We have revealed a natural reaction. The dream is to utilize our discoveries as a way to create a catalyst to contribute to the production of fertilizers for food security.”