Strain-specific surface polysaccharides mediate bacterial induction of metamorphosis in the coral Pocillopora acuta

Microbial biofilms induce larval settlement and metamorphosis in marine invertebrates, including corals, but the specific molecular cues for corals remain unclear. Glycosylated lipids and polysaccharides from crustose coralline algae, along with bacterial small molecules, are proposed inducers, but their effects vary by coral species, bacterial strain, and environment, highlighting the need to clarify their molecular basis and specificity. Here, we identify bacterial lipopolysaccharide (LPS), specifically its O-antigen polysaccharide component, as a settlement cue for larvae of the coral Pocillopora acuta. Using purified LPS from diverse marine bacteria, we show that inductive activity is bacterial species and strain specific and correlates with the structure of O-antigen. These findings support a glycan-centered mechanism of coral recruitment, implicating microbial surface chemistry as a key determinant of larval settlement. By demonstrating a specific and functional role for O-antigen, we expand the ecological relevance of bacterial glycans beyond immunity and pathogenesis, highlighting their role in key developmental and ecological transitions in marine invertebrates. As coral reefs face escalating environmental pressures, employing such signals offers a promising strategy to support reef restoration.