Comparative meta transcriptomics uncovers the physiological responses of zooplankton to nutrient heterogeneity in their living environment

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 zooplankton, the chemo diversity and properties of this DOM, along with the physiological mechanisms influencing these characteristics in the environment, remain inadequately explored. We conducted zooplankton sampling followed by onboard DOM release experiments in heterogeneous estuarine-coastal water systems, followed by molecular characterization of the DOM using Fourier-transform ion cyclotron resonance mass spectrometry. Additionally, we analyzed zooplankton metabolic activities through meta-transcriptomics to elucidate the relationship between the chemical properties of the released DOM and the underlying physiological processes of zooplankton. Our findings reveal substantial variations in the molecular diversity of DOM released by zooplankton, more specifically lipids-like, protein-like, and unsaturated hydrocarbon-like between mesotrophic and eutrophic coastal zooplankton communities. We found strong correlations between chemical composition of the DOM and zooplankton gene functions associated with metabolism processes such as carbohydrate metabolism, nucleotide processing, energy production, and coenzyme metabolism. Furthermore, the modified aromaticity indexes of the released DOM are also highly associated with metabolism-related gene functions such as amino acid, carbohydrate metabolism, lipid, energy production, as well as glycan biosynthesis, indicating that zooplankton metabolic processes significantly influence DOM aromaticity. This study enhances our understanding of how organisms metabolic processes shape the molecular characteristics of DOM they release, highlighting its implications for nutrient cycling in the environment.