Fig. 2: Conceptual illustration of an indicator of compositional-level thermal responses (iCER) and the expected mechanisms of functional reorganization of DOM under climate change.

a Calculation of molecule-specific thermal responses (MER) of DOM. MER is defined as the effect size estimated from correlating each molecule’s relative abundance with temperature in a region and/or temporal period. Effect size can be measured using statistical methods such as the Spearman’s correlation coefficient ρ. Two examples are shown for positive (red) and negative (blue) thermal responses, which indicate warm-accumulating and warm-depleting molecules, respectively. b Compositional-level thermal responses (iCER) of DOM under climate change. The iCER values, indicated by black points, are calculated as the average of relative abundance-weighted MERs of multiple DOM molecules in a region and/or temporal period. The pie chart describes the hypothesized functional reorganization of DOM composition along the temperature gradient from being dominated by warm-depleting to warm-accumulating molecules. Pie sections of a circle indicate the relative abundance of molecules with positive (red) and negative (blue) thermal responses (i.e., Spearman ρ). An iCER value of 0 indicates a perfect equivalence between the two groups of molecules with positive and negative MERs. An example of functional reorganization under warming in a DOM assemblage is shown in c. Specifically, a DOM pool consists of diverse molecules (gray circles) that can be assembled into a local assemblage by biological and environmental filtering, such as temperature. A molecule that accumulates in a DOM assemblage at high temperatures will respond positively to warming (red circles) in contrast to a molecule depleted at high temperatures (blue circles). The shading of circles from light to dark represents the magnitude of thermal responses from low to high, respectively. The intrinsic molecular traits (e.g., thermodynamic properties) of these molecules are hypothesized to determine the functional reorganization of DOM composition with temperature change. That is, the thermal responses of DOM assemblages may increase towards warmer sites, where DOM molecules may become thermodynamically less favorable (i.e., higher Gibbs free energy for the half reaction of organic carbon oxidation) and with lower biodegradability (“D”).