Dynamic physiology broadens the molecular composition of kelp-derived dissolved organic matter exported to the coastal ocean

Chance J English, Tilman Schramm, Keri Opalk, David A Siegel, Daniel Petras, Craig A Carlson

Metabolites are exuded by marine primary producers and contribute to the pool of dissolved organic matter that structures the ecology and biogeochemistry of marine ecosystems. In the coastal ocean, macroalgae such as kelps fix inorganic carbon via photosynthesis and release a fraction of that carbon as dissolved organic matter. In this study, we characterized the exo-metabolome of the globally distributed foundation species Macrocystis pyrifera (giant kelp) using liquid chromatography-tandem mass spectrometry. We assessed the exuded metabolites across gradients in giant kelp physiological state and productivity driven by seasonal, environmental, and intrinsic age-dependent biological factors. Our results showed that kelp physiological condition is a main driver of exudate composition, as revealed by changes in the exudate stoichiometry and the relative abundance of molecular families. Notably, we identified that giant kelp may be an important source of brominated dissolved organic matter to the coastal ocean during senescence, with exudate bromine to carbon molar ratios (Br:C) reaching 0.14. Lastly, we found that roughly 20% of giant kelp dissolved organic carbon is recalcitrant to microbial remineralization. Surprisingly, we found no significant variability in this recalcitrance across giant kelp physiology, despite large shifts in its DOM composition. This suggests that a diverse mixture of kelp-derived metabolites, including peptides, brominated phenols, and eicosanoids, may contribute to long-term carbon storage.