Marine macroalgae and their associated bacterial communities affect larval settlement and survivorship of the coral Pocillopora damicornis.

Macroalgae play crucial roles as major habitat-forming organisms in marine ecosystems, having significant impacts on coral recruitment and reef recovery. However, the interactions between marine macroalgae and coral larvae remain poorly understood. Furthermore, little is known whether differences in bacterial assemblages associated with macroalgae may play roles in this process. Here, we comprehensively investigated the impacts of different macroalgae and their associated microbiomes on larval settlement and survival of coral Pocillopora damicornis. The results revealed significant variations in larval settlement and survival rates when exposed to different macroalgal species. The highest settlement rate, reaching 90%, was observed in the presence of the red alga Hypnea pannosa, followed by green algae Caulerpa serrulata, C. racemosa, and brown algae Turbinaria gracilis, Sargassum polycystum. Correspondingly, similarities in bacterial compositions were observed between H. pannosa and C. racemosa, as well as between T. gracilis and S. polycystum, implying associated bacterial may be related with the algal functions. Furthermore, macroalgae that facilitate larval settlement exhibited higher abundances of amplicon sequence variants (ASVs) associated with the metabolism of dimethylsulfoniopropionate or the antagonism of known coral pathogens. However, the brown alga Padina boryana failed to induce larval settlement with survival rate of zero after 120 h. The algal species harbored more abundances of ASVs related to Rhizobiaceae. These findings highlight the significant impact of macroalgae and their associated microbiomes on coral recruitment, as they influence both larval settlement and survival rates.