CAMBRIDGE, MD (April 29, 2024)—When a leaf falls in the forest and decomposes, is it still a leaf when it washes into a river? An international team of researchers from the University of Maryland Center for Environmental Science, Linköping University, Sweden, and the Helmholtz Zentrum in Munich now understand how biomolecules in nature are transformed to yield complex natural organic matter found in rivers and lakes, a dramatic chemical transformation process that has confounded researchers for over half a century. Their study has been published in the journal Nature.
"The chemical diversity of natural dissolved organic matter in lakes and rivers is extreme and yet most of it does not resemble molecules produced by organisms," said Michael Gonsior, professor and biogeochemist from the University of Maryland Center for Environmental Science and a co-author of this study. "We finally discovered a credible mechanism in how biomolecules are transformed in nature yielding extreme complexity and explaining why most organic molecules found in lakes and rivers are not very reactive. We termed the mechanism oxidative dearomatization or ODA."
When a leaf detaches from a tree and falls to the ground, it consists of a few thousand distinct biomolecules that can be found in most living matter. As it begins to decompose, things change. Insects and microorganisms consume the leaf, and sunlight and humidity cause further breakdown. Eventually, the molecules from the decomposed leaf are washed into rivers, lakes, and oceans. However, the decomposition process has turned those thousands of known biomolecules into millions of molecules that look very different and have very complex and largely unknown structures.
Until now, this dramatic chemical transformation process has remained a mystery that has confounded researchers for over half a century. The researchers discovered that a specific type of reaction, known as oxidative dearomatization, is behind the mystery. The reaction has long been studied and applied extensively in pharmaceutical synthesis, yet its natural occurrence has remained unexplored.
"Now we can elucidate how a couple of thousand molecules in living matter can give rise to millions of different molecules that rapidly become very resistant to further degradation", said corresponding author Norbert Hertkorn, affiliated with the Helmholtz Zentrum, Munich, Germany and Linköping University, Sweden.
In this study, the researchers showed that oxidative dearomatization changes the three-dimensional structure of some biomolecule components, which in turns can activate a cascade of subsequent and differentiated reactions resulting in millions of diverse molecules. Previously, scientists believed that the path to dissolved organic matter involved a slow process with many sequential reactions. However, this study suggests that transformation and complexification may occur relatively quickly.
The researchers examined dissolved organic matter from four tributaries of the Amazon River and two lakes in Sweden. They employed a technique called nuclear magnetic resonance (NMR) to analyze the structure of millions of diverse molecules. Remarkably, regardless of the climate, the fundamental structure of the dissolved organic matter remained the same.
"A key behind the findings was unconventional use of NMR [nuclear magnetic resonance] in ways allowing studies of the deep interior of large dissolved organic molecules—thereby mapping and quantifying the chemical surroundings around the carbon atoms," said Siyu Li, researcher at the Helmholtz Zentrum Munich, Germany, and lead author of this study.
The researchers were surprised by the high fraction of carbon atoms that were bound to three other carbon and one oxygen atom in the dissolved organic matter, something that is unusual in biomolecules.
"This renders the organic matter stable, allowing it to persist for long time and preventing it from rapidly returning to the atmosphere as carbon dioxide or methane," said co-author David Bastviken, professor in environmental change at Linköping University, Sweden.
The study received essential funding from the Alexander von Humboldt Foundation, the Swedish Research Council, Formas, and the European Research Council.
The study "Dearomatization drives complexity generation in freshwater organic matter" was published in Nature.
