Accurate Accounting and Molecular Composition Analysis Reveal Sargassum Beds as a Major Marine Carbon Sequestration

Marine carbon sequestration is increasingly recognized as a potential contributor to mitigate global warming. Recent studies have demonstrated that macroalgae possess a significant carbon sequestration potential. In this study, the biomass dynamics of the Sargassum beds in the Weizhou Island of China was investigated throughout their entire life cycle. Laboratory experiments revealed that dissolved organic carbon (DOC) accounts for 17.95% of the Sargassum carbon stock, of which 64.29% was converted into recalcitrant DOC (RDOC). By integrating biomass data, the carbon sequestration of RDOC produced by the Sargassum beds was estimated to be 9.17 g C m^-2 yr^-1. In addition, the number of RDOC molecules directly released by Sargassum spp. far exceeded those formed by microbial transformation (999 vs 265). Excitation-emission matrix (EEM) fluorescence spectroscopy revealed that the Sargassum growth and microbial activities both produce refractory humic-like substances that may serve as RDOC proxies. And the enrichment of humic substances is driven by Sargassum metabolism and the microbial humification process. By elucidating the formation mechanism of macroalgae RDOC and establishing a more precise method for calculating the RDOC carbon stock, this study reveals the important role of macroalgae in carbon sequestration and provides novel insights for quantifying the macroalgal carbon sequestration.