Natural minerals catalyze phosphorus cycling

Phosphorus is a key element for all life on Earth. For decades, researchers thought that, in nature, only enzymes could transform organic phosphorus—phosphates within biomolecules—into its bioavailable, inorganic form, free phosphate ions. Minerals in sediment and soil were thought to only adsorb phosphorus, not participate in the dephosphorylation reaction. But now new research shows that naturally occurring iron oxide minerals also act as catalysts (Nat. Commun. 2024, DOI: 10.1038/s41467-024-47931-z).

The work began years ago, when lead researcher Ludmilla Aristilde, an environmental chemist at Northwestern University, designed an initial experiment to follow the dephosphorylation products that developed when adenosine triphosphate was mixed with pure ferrihydrite, an iron oxide mineral commonly found in soil. Using high-resolution mass spectrometry, the team was able to search for more than just phosphorus, the only target in past experiments. This expertise paid off; the team found organic products—adenosine diphosphate, adenosine monophosphate, and adenosine—but no inorganic phosphate. “If you were only following phosphate,” Aristilde says, “you would say there is no catalysis,” and might incorrectly assume iron oxides are noncatalytic.

A serendipitous encounter with Sharon Bone, a former lab mate and now beamline scientist at SLAC National Accelerator Laboratory, helped Aristilde search for the missing phosphate. Bone agreed to collaborate and initiated an urgent request for beam time so the researchers could analyze the surface of the ferrihydrite with advanced X-ray scattering techniques. Not only did they find phosphate adsorbed to the pure mineral, Aristilde says they also found even more organic, dephosphorylated products. Their findings indicated that the pure minerals were clearly acting as catalysts.

But other researchers remained unconvinced after Aristilde published the work in 2019 (J. Colloid Interface Sci. 2019, DOI: 10.1016/j.jcis.2019.03.086). Fellow scientists asked her, What if real-world samples are too complex to behave in the same way? So Aristilde searched for more collaborators. Two field scientists sent her samples: sediment from the bottom of a lake and soil from a forest floor. Both contained a fraction of iron oxide minerals.

With those real-world samples, Aristilde and her team repeated their initial experiments. After they analyzed the mass spectra and X-ray data, the role of minerals was clear. “Iron oxide in a soil matrix and sediment matrix is also acting as a catalyst,” Aristilde says. And not only are the minerals catalyzing dephosphorylation, she says, but the rate of mineral catalysis is comparable to that of an enzyme.

Finding abiotic pathways of producing bioavailable phosphorus in the natural environment has important implications for how researchers understand the phosphorus cycle, says Elizabeth Herndon, an environmental geochemist at Oak Ridge National Laboratory. “With some of the quantitative information that they provide here,” she says, “there’s the potential to incorporate mineral-catalyzed pathways into biogeochemical models.”

Herndon thinks there are plenty more ecosystems to investigate. “This study opens the door to looking at this catalysis across more environments,” she says, “Maybe it’ll be important in some places and less in other places.”