S-containing molecular markers of dissolved organic carbon attributing to riverine dissolved methane production...

The emission of methane (CH₄) from streams and rivers contributes significantly to its global inventory. The production of CH₄ is traditionally considered as a strictly anaerobic process. Recent investigations observed a "CH₄ paradox" in oxic waters, suggesting the occurrence of oxic methane production (OMP). Human activities promoted dissolved organic carbon (DOC) in streams and rivers, providing significant substrates for CH₄ production. However, the underlying DOC molecular markers of CH₄ production in river systems are not well known. The identification of these markers will help to reveal the mechanism of methanogenesis. Here, Fourier transform ion cyclotron mass spectrometry and other high-quality DOC characterization, ecosystem metabolism, and in-situ net CH₄ production rate were employed to investigate molecular markers attributing to riverine dissolved CH₄ production across different land uses. We show that endogenous CH₄ production supports CH₄ oversaturation and positively correlates with DOC concentrations and gross primary production. Furthermore, sulfur (S)-containing molecules, particularly S-aliphatics and S-peptides, and fatty acid-like compounds (e.g., acetate homologs) are characterized as markers of water-column aerobic and anaerobic CH₄ production. Watershed characterization, including riverine discharge, allochthonous DOC input, turnover, as well as autochthonous DOC, affects the CH₄ production. Our study helps to understand riverine aerobic or anaerobic CH₄ production relating to DOC molecular characteristics across different land uses.