Thermodynamic property and equilibrium state drive the spatial pattern of dissolved organic matter refractory in global ocean

Marine dissolved organic matter (DOM) has been studied for decades in understanding of its compositions and cycling. Advances in analytical techniques have revealed that marine DOM is a complex mixture of thousands of molecules. Two theories, concentration threshold and molecular composition, provide insights into DOM cycling in the global ocean, either separately or in conjunction. This study integrates four groups of incubation experiments with 1,104 DOM samples collected from across the global ocean to calculate the thermodynamics and chemical equilibrium state of each individual DOM formula, utilizing molecular composition data obtained from Fourier transform ion cyclotron resonance mass spectrometry. Our findings indicate that marine DOM transitions from a thermodynamic nonequilibrium state to an equilibrium state during the degradation process. In addition, refractory DOM was found to be a group of molecules that have approached a relative equilibrium state, leading to its bulk stability. In-house incubation experiments, observations from the open ocean water column and the global conveyor belt further consolidate this finding. We conclude that the transformation of marine DOM is influenced by both concentration and composition, which together determine its thermodynamic properties, reactivity, and refractory characteristics in the global ocean.