Species-specific metabolomic profiles of coral reef coralline algae and their influence on the larval settlement of corals and crown-of-thorns starfish

To address a significant knowledge gap in chemical ecology underpinning larval settlement processes on coral reefs, we examined the tissue-associated metabolomes using nuclear magnetic resonance (NMR) spectroscopy across 14 species of crustose coralline red algae (CCA) and one non-coralline calcareous red alga collected from the Great Barrier Reef, Australia. We further explored the relationship between algal metabolites and the settlement success of fifteen reef-building coral species across five families, as well as a key coral predator, the crown-of-thorns starfish (CoTS; Acanthaster cf. solaris). We found that algal metabolomes are highly variable and differ among species, phylogenetic lineages, and reef habitats, highlighting the combined influence of evolutionary history and environmental context on algal metabolomes. We also identified strong, positive correlations between specific algal metabolites, particularly disaccharides and trisaccharides (e.g., raffinose, maltose), and glycine betaine with high settlement success in both corals and CoTS. This study provides the most comprehensive analysis to date of coralline algal metabolomes and their ecological significance to coral reef ecosystems. These results provide novel chemical, biological and ecological insights that may be used to inform the optimisation of coral aquaculture techniques for reef restoration, as well as potential strategies for controlling CoTS outbreaks to mitigate ongoing reef decline.