Microbial decomposition of Sargassum enhances recalcitrant dissolved organic carbon production and sedimentary carbon...

Seaweeds are fundamental components and primary producers in marine ecosystems, and their growth and decay significantly influence environmental conditions and microbial community structure. However, considerable debate persists regarding whether seaweed decomposition enhances marine carbon sequestration, and the role of microbes in seaweed degradation remains unclear. This study examined the degradation process of Sargassum under controlled laboratory conditions to assess its potential for carbon sequestration. After 240 days of decomposition on the sediment surface, the dissolved organic carbon (DOC) in the overlying water stabilized, with approximately 24.79 ± 11.42 % having been transformed into recalcitrant dissolved organic carbon (RDOC), accounting for about 1.37 ± 0.12 % of the initial seaweed detrital carbon. The decomposition rate of Sargassum buried in sediments was significantly slower (p < 0.05), leading to an estimated 22.15 ± 3.49 % increase in carbon sequestration. The RDOC released by Sargassum accounted for approximately 10.91 %-11.78 % of the total RDOC molecules. Microbial degradation of Sargassum produced RDOC molecules enriched in CHOS and CHONS, with larger molecular mass, higher double bond equivalents (DBE), and greater aromaticity indices (AI), all consistent with recalcitrant molecular properties. Microbial succession during Sargassum degradation enriched chemoheterotrophic and fermentative taxa, thereby driving the conversion of labile DOC (LDOC) to RDOC. These results demonstrate that Sargassum decay enhances RDOC accumulation and sedimentary carbon sequestration. Therefore, a future management strategy for senescing Sargassum could involve burying some in nearshore sediments to increase carbon storage and mitigate pollution from decomposition in coastal environments.