Deciphering Microbially Driven Labile and Refractory Molecular Candidates in Dissolved Organic Matter

Microbial activities drive the cycling of dissolved organic matter (DOM) from labile to refractory states, thus contributing to the long-term carbon sequestration in the ocean. However, due to the intricate molecular composition of DOM, identifying indicators of microbially related DOM remains a challenge. In this study, we propose molecular candidates for bio-labile (n = 537) and bio-refractory (n = 1,025) formulas, which were discerned through incubation experiments using ultrahigh-resolution mass spectrometry. Bio-labile formulas exhibited greater hydrogenation, whereas bio-refractory formulas comprised oxidized, unsaturated and aromatic molecules with higher molecular weight. Bio-candidates, in contrast to photo- or terrestrial-related counterparts, dominated molecular composition by higher relative intensity. The application of these molecular candidates facilitated the tracing of molecular distribution and transformation patterns across large-scale aquatic environmental gradients. This molecular identification framework offers insights into resolving microbially mediated molecules and advancing our understanding of biologically related DOM at the molecular level.