Bacillus pumilus SG44 Reshapes the Microbiome and Alters Metamorphosis and Development of Aurelia coerulea Polyps

Planula settlement and metamorphosis are critical transitions in the life cycle of the moon jellyfish Aurelia coerulea, and associated microorganisms can influence these processes. We developed a co-culture system combining the marine bacterium Bacillus pumilus SG44 with A. coerulea planula larvae and, using 16S rRNA gene high-throughput sequencing, examined how strain SG44 affects larval settlement and metamorphosis and the associated microbiota. Treatment with strain SG44 significantly elevated the settlement and metamorphosis rate of planulae, although the resulting polyps exhibited morphological deformities. Strain SG44 became established as the dominant population in both polyp tissues and the surrounding culture seawater. Strain SG44 significantly decreased the diversity of the polyp-associated microbial community and markedly reduced the relative abundance of Neptuniibacter, a reported beneficial taxon in jellyfish-associated microbiota, as well as other putatively beneficial genera, including Halomonas and Sulfitobacter. Co-occurrence network analysis revealed that the network of treatment with strain SG44 contracted in size while maintaining a comparably high average degree. These results indicate that strain SG44 affects the development of A. coerulea larvae, an effect that may be linked to shifts in the host-associated microbial community, although a causal relationship remains to be established, and provide a new perspective on microbe–host interactions during scyphozoan early development.