A comparative analysis of bacterial community characterization and host–bacteria interactions between bi-macroalgal blooms caused by Ulva prolifera and Sargassum horneri

Macroalgal blooms have increasingly occurred in coastal regions worldwide. Since 2017, simultaneous green tides (<i>Ulva prolifera</i>) and golden tides (<i>Sargassum horneri</i>) have recurred annually in the Yellow Sea, forming a unique large-scale bi-macroalgal bloom. Interactions between macroalgae and their associated bacterial communities are recognized as key ecological drivers of algal bloom dynamics. In this study, the differences in phycosphere-associated bacterial communities and algae-derived metabolites between <i>U. prolifera</i> and <i>S. horneri</i> were explored using the 16S rRNA amplicon combined with broad-spectrum metabolomics. The results reveal that the diversity of phycospheric and epiphytic bacterial communities of <i>S. horneri</i> is significantly higher than that of <i>U. prolifera</i>. We observed distinct phycosphere bacterial recruitment between the two macroalgal species. Verrucomicrobiae were the stable core microbiota in the <i>U. prolifera</i> phycosphere, whereas Gammaproteobacteria and Bacteroidia represented the core members in that of <i>S. horneri</i>. Community assembly analyses indicate that deterministic processes predominantly shape the epiphytic bacterial communities, suggesting strong host selection effects. Metabolomic profiling further revealed that the metabolites secreted by <i>U. prolifera</i>, such as phenolic acids and organic acids, promote the proliferation and colonization of <i>Verrucomicrobiae Rubritalea</i>, which may enhance the stress tolerance of the host. In contrast, the amino acids, nucleotides, lipids, and their derivatives are key metabolites that promote the colonization of Gammaproteobacteria <i>Vibrio</i> and <i>Marinomonas</i> on the <i>S. horneri</i> surface, which may inhibit host growth through the production of algicidal substances. Together, these results suggest that <i>U. prolifera</i> and <i>S. horneri</i> can secrete different metabolites that recruit epiphytic microbial communities and influence macroalgae-bacteria interactions. These findings provide insights into the ecological mechanisms underlying host-bacteria interactions and their roles in the formation and persistence of macroalgal blooms.