Unravelling the role of crustose coralline algae microbiomes on coral larval settlement in the Great Barrier Reef

Crustose coralline algae (CCA) enhance coral recruitment, but the response of coral larval settlement to CCA varies between CCA species. Furthermore, it is unclear whether coral larvae respond to settlement cues from the algal host itself or its associated microorganisms. To determine whether CCA-derived settlement cues have a microbial origin, we interrogated the microbiome of 14 coralline algal species and a calcareous non-coralline alga eliciting varying levels of settlement across 14 coral species from a wide diversity of families found in the Great Barrier Reef. Linear regression, differential abundance, indicator species, and random forest analyses were used to identify microbial taxa associated with high or low coral settlement. We found that the relative abundance of specific microbial amplicon sequence variants (ASVs) correlated with settlement and that these responses were largely coral species-specific. A select few microbial taxa associated with high or low settlement were shared across the corals Dipsastrea favus , Echinophyllia aspera, Lobophyllia corymbosa, Mycedium elephantotus , and Platygrya sinensis , suggesting potential shared settlement or inhibition cues. While shared ASVs associated with high coral settlement were found across multiple CCA species, low settlement AVSs were confined to few low settlement CCA species. Candidatus Nitrosopumilus and Filomicrobium microbes were found as potential shared microbial inducers, and members of Pirellulaceae and Flavobacteriaceae were identified as potential settlement inhibitors. These findings contribute to our growing knowledge of potential coral larval settlement cues and provide deeper insights into the link between the CCA microbiomes and coral recruitment.