Effects of Sargassum beds on carbon sequestration and microbial community structure in the adjacent sediment cores

Seaweed detritus transported to the deep sea is an important component of marine carbon sequestration. However, the impacts of seaweed decomposition on adjacent sediments and the response mechanisms of microbial communities are still unclear. In this study, we analyzed the changes in carbon and nitrogen contents, the molecular composition of dissolved organic matter (DOM), and microbial communities in three sediment cores at different distances (1, 2, and 3 km, respectively) from Sargassum beds in Weizhou Island coastal waters, China. The results show that the impact of Sargassum on sediment cores exhibits strong spatial heterogeneity, with more aromatic-like and tannin molecules found at the nearest site (S1). Compared to the other two sediment cores, the microbial community composition of S1 is significantly different. The homogeneous selection dominates the assembly of the microbial community at S1. In addition, the input of decomposing organic matter from Sargassum contributed to a significant increase in the abundance of functional genes related to carbon and nitrogen decomposition at S1, leading to a significant decrease in TOC and TN content in the sediments. Our study demonstrated that the decomposition of Sargassum increased the sources and molecular species of organic matter in the adjacent sediments and stimulated microbial decomposition of organic matter. Strengthening the protection of Sargassum beds contributes to improving biodiversity and promoting the marine carbon and nitrogen cycles.