The short-term effects of ocean acidification on the epiphytic bacterial community of Sargassum thunbergii via high-throughput sequencing.

Marine macroalgae and their epiphytic bacteria have established a symbiotic relationship. Although the effects of ocean acidification (OA) on macroalgae have been extensively studied, its impact on these epiphytic bacteria remains unclear. This study investigated the OA-induced shifts in the epiphytic bacterial community of Sargassum thunbergii from Qingdao's intertidal zone using 16S rDNA sequencing. The results indicated that elevated CO₂ altered bacterial community structure and function, reducing diversity while maintaining dominant taxa but significantly changing their relative abundances. The abundances of Proteobacteria, Firmicutes, and Verrucomicrobiota declined, whereas Campylobacterota, Desulfobacterota, and Spirochaetota increased. The specific phyla like Cloacimonadota, Calditrichota and Entotheonellaeota also emerged. Based on the analysis of the characteristics of these altered bacterial taxa, it is speculated that these shifts were linked to the environmental adaptability and stress resistance of epiphytic bacteria as well as the metabolic activities of the host algae. Functional predictions revealed that OA primarily affected nitrogen and sulfur metabolism in the epiphytic bacterial community, with effects intensifying over time. Specifically, nitrogen fixation increased, while dark oxidation of sulfur compounds, dark sulfite oxidation, and dark sulfur oxidation decreased. These results suggest that ocean acidification may influence epiphytic bacterial communities through two potential pathways: it could induce abundance changes in bacterial taxa with varying stress resistance and adaptability, while potentially promoting shifts in bacterial taxa closely associated with host algal metabolic activities, which may ultimately lead to restructuring of the epiphytic bacterial community on S. thunbergii. These findings provided new insights into the macroalgae-epiphytic bacteria interactions under ocean acidification and provided important guidance for macroalgal cultivation.