Zooplankton Metabolism Shapes Molecular Composition of Dissolved Organic Matter in Coastal Waters

Dissolved organic matter (DOM), the largest reservoir of organic material in the ocean, plays a crucial role in the global nutrient cycle and the microbial loop. While existing studies have documented significant DOM release by marine organisms, how organismal metabolic processes under different nutrient levels influence the intrinsic characteristics of their derived DOM remains largely unknown. We conducted zooplankton DOM release experiments in estuarine-coastal water systems, followed by molecular characterization of the DOM using Fourier transform ion cyclotron resonance mass spectrometry and metabolic profiling using community-level gene expression analysis (metatranscriptomics). Our results reveal substantial differences in the molecular composition and characteristics of zooplankton-derived DOM from eutrophic compared to mesotrophic communities. Moreover, zooplankton in mesotrophic environments exhibited a higher expression of metabolic genes. We found strong correlations between the zooplankton-derived DOM chemical composition and zooplankton gene expression associated with metabolic processes including carbohydrate metabolism, amino acid, energy production, lipid, terpenoid-polyketide and glycan biosynthesis. Furthermore, the modified aromaticity indexes of the zooplankton-derived DOM are also highly associated with metabolism-related gene functions, suggesting that metabolic responses may drive the derived DOM aromaticity characteristics. This study enhances our understanding of how organismal metabolic activities shape the molecular properties of released DOM, underscoring their critical roles in biogeochemical cycling.