A riddle that eluded the scientific world for over 50 years has now been answered. Researchers from, among others, Linköping University and Helmholtz Zentrum München have shown that a special type of chemical reaction can explain why organic matter in lakes and waterways is so resistant to decomposition. The study is published in the journal Nature.
– At a time when the carbon cycle is driving global change, we have found an explanation for why so much carbon can be stored on the planet away from the atmosphere for a long time, says Alex Enrich Prast, professor at the Department of Environmental Change, Linköping University.
But we take it from the beginning. When a leaf comes off a tree and falls to the ground, it begins to decompose immediately. Before the leaf breaks down, it consists of a few thousand well-known molecules, roughly the same molecules found in all living matter.
The decomposition of the leaf undergoes several phases. Insects and microorganisms start to eat it, the sun’s rays and the humidity of the air affect the leaf and it breaks down more and more. Finally, the molecules of the decomposed leaf are washed out into rivers and into lakes or into the sea.
But by now the thousands of known molecules have turned into millions of different molecules with complex and unknown structures. Then the leaf has turned into something called dissolved organic matter. Until now, this chemical transformation process has been an enigma that has eluded scientists for over half a century.
– It has been the holy grail in my field of research for over 50 years. Now we can explain how a thousand molecules in living matter can give rise to millions of different molecules that quickly become very resistant to further degradation, says Norbert Hertkorn, researcher at Tema Mjöfärndärning and formerly at Helmholtz Zentrum München.
What the researchers discovered is that a type of reaction called oxidative dearomatization lies behind the mystery. It is a well-studied reaction that is widely used in pharmaceutical production but has so far been unexplored in nature.
In the study, the researchers showed that oxidative dearomatization changes the three-dimensional structures of certain parts of biomolecules, which in turn can activate a cascade of different subsequent reactions, resulting in millions of different molecules.
Previously, scientists believed that the path to dissolved organic matter has a slow process with many reactions that follow one another. But the study indicates that the transformation instead takes place relatively quickly.
The researchers investigated dissolved organic matter in four tributaries to the Amazon River and two medium-sized lakes in Sweden. They used a measurement method called nuclear magnetic resonance that analyzes the structure of millions of different molecules. Remarkably, the basic structures of the molecules were the same regardless of climate. It is the first time nuclear magnetic resonance has been used in this way and provides a unique insight into the composition of dissolved organic matter.
In biomolecules, carbon atoms can be linked to four other atoms, usually to hydrogen or oxygen. But the researchers were surprised by the fact that a very high percentage of the organic carbon atoms were not linked to any hydrogen but instead mainly to other carbon atoms. Of particular interest was the large number of carbon atoms specifically bonded to three other carbons and one oxygen atom, a structure that is very unusual in biomolecules and that points to the importance of the oxidative dearomatization.
According to David Bastviken, professor at Theme Environmental Change at Linköping University, these surprisingly common structures are very difficult for microorganisms to break down. This in turn prevents the carbon from returning to the atmosphere in the form of carbon dioxide or methane gas.
– If we understand this process, we can also gain a deeper understanding of how the world’s major carbon sinks actually work and what role they have in the carbon cycle, and by extension how they affect the climate, says David Bastviken.
The study was mainly funded by the Alexander von Humbolt Foundation, the Swedish Research Council, Formas and the European Research Council (ERC).
The article: Dearomatization drives complexity generation in freshwater organic matter, Siyu Li, Mourad Harir, David Bastviken, Philippe Schmitt-Kopplin, Michael Gonsior, Alex Enrich-Prast, Juliana Valle, Norbert Hertkorn; Nature 2024published online 24 April 2024. DOI: 10.1038/s41586-024-07210-9
Contact
David Bastviken, professor, [email protected], 013-28 22 91
Alex Enrich Prast, professor, [email protected], 013-28 10 23
Norbert Hertkort, researcher, [email protected]
The press release sent by:
Anders Törneholm
press contact, Linköping University
013-28 68 39
[email protected]
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