The NOSC-ΔG relationship in DOM: why it works, why it fails

Dissolved organic matter (DOM) regulates carbon storage and microbial activity across aquatic ecosystems, yet we still lack a clear mechanistic link between its molecular composition and thermodynamic favourability and persistence. We combine quantum chemistry with a structure informed Random Forest model to predict the Gibbs free energy of oxidation per mole of carbon for more than 4000 environmentally relevant molecules, and apply these predictions to ultra high resolution mass spectrometry data from standard DOM reference materials. By revisiting the relationship between the nominal oxidation state of carbon (NOSC) and Gibbs free energy, we show that NOSC captures first order trends across broad DOM quality gradients but fails within individual molecular formulae and NOSC classes, where structurally similar molecules can differ by more than 100 kJ molC⁻¹ in predicted gibbs free energy. These results link DOM chemodiversity to thermodynamic constraints on microbial use and help to explain why DOM fractions with similar elemental composition can follow divergent degradation pathways across aquatic redox gradients due to underlying structural and thermodynamic diversity.

Presentation Preference: Standard Oral (12 Minutes)

Primary Presenter: Thomas Sibille, Technische Universität Bergakademie Freiberg (thomas-peter.sibille@mineral.tu-freiberg.de)

Authors:

Maximilian Lau, Technische Universität Bergakademie Freiberg  (maximilian.lau@ioez.tu-freiberg.de)

Klaus Knorr, Universität Münster  (kh.knorr@uni-muenster.de)

Oliver Lechtenfeld, Helmholtz Centre for Environmental Research GmbH - UFZ  (oliver.lechtenfeld@ufz.de)