Global dispersal of macroalgae-derived recalcitrant DOC may substantially contribute to deep-ocean carbon storage.

Macroalgae, the dominant primary producers of Earth's most productive coastal vegetated ecosystems and a vital component driving ocean carbon sequestration, produce ∼5 to 80% of their net primary productivity as dissolved organic carbon (DOC). However, the global extent, fate, and contribution of this macroalgae-derived DOC to the pelagic carbon pool remain poorly understood. We establish a molecular fingerprint of the dominant coastal macroalga Saccharina japonica-derived DOC (kelp-derived DOC) using ultrahigh-resolution mass spectrometry and conservative filtering. We differentiate the kelp-derived DOC into labile (LDOC) and recalcitrant fractions (RDOC) based on prolonged resistance to microbial degradation, identify discriminating molecular traits through machine learning, and integrate them into a stability metric, molecular recalcitrance score (MRS). Both candidate kelp-associated RDOC signals and the more persistent, semilabile subset of candidate kelp-associated LDOC signals are detected throughout the global ocean, as far as ∼10,000 kilometers from kelp ecosystems and at depths of ∼5500 meters. Detected candidate kelp-associated DOC signals exhibit elevated MRS, with values positively correlated with distance from source regions, indicating selective long-distance transport of intrinsically more stable compounds. We preliminarily estimate that coastal macroalgal ecosystems globally may contribute 320.7 to 481.1 teragrams of carbon per year of RDOC. The detection of candidate kelp-associated RDOC signals throughout the global ocean provides molecular-level evidence of the long-distance export of macroalgae-derived RDOC to the pelagic and deep ocean, highlighting a major, previously unrecognized pathway of long-term storage of macroalgae-derived DOC in global oceans, although the precise fraction exported to the deep ocean remains to be quantified.