Particulate (POC) and dissolved (DOC) organic carbon released during macroalgae cultivation has been shown to lead to carbon burial and export to the open ocean and deep sea. The recalcitrant portion of the DOC (RDOC) is a critical contributor to carbon sequestration and a major knowledge gap. We investigate the production and bioavailability of DOC released from Saccharina latissima, a primary cultivar in Europe, with a focus on RDOC quantification. DOC release rates were 0.5-1.4 mg C g(dw)-1 h-1 within 24 h, and concentrations stabilized after 187 days of biodegradation, at 37-79% (mean 58%) from initial levels. The kelp released 920 (17%) unique compounds, including large quantities of sulfur-containing molecules (CHOS, CHONS). Increasing recalcitrance was observed in kelp and control treatments, despite small variations in CRAM molecules. Microbial biodegradation generated 1,805 (34%) new molecules, including 1,207 in the recalcitrant DOM fraction. Island of Stability (IOS) analysis revealed an intensity-weighted abundance of 0.2% RDOC in kelp treatments, with persistence on geological time scales. Beyond its biomass applications, macroalgae cultivation enables scalable CO2 sequestration in the oceans through RDOC production from continuous release and degradation of DOC and POC during grow-out, in itself a value-added product.
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