High fidelity and lack of spatial variation in coral-algal symbiont associations in the world’s southernmost coral reef

Reef-building corals form essential symbioses with Symbiodiniaceae, a diverse family of photosynthetic dinoflagellates. Characterizing this association is critical because it greatly influences the physiology of the host. To this end, we aimed to document the coral-algal associations on a high-latitude reef to detect potentially endemic or flexible associations. Using a gel-based genetic analysis, this study characterized the dominant Symbiodiniaceae genera in four coral species: Acropora lovelli, Isopora cuneata, Porites heronensis, and Seriatopora hystrix, at six sites around Lord Howe Island (LHI)—the world's southernmost coral reef. Genetic analysis revealed that symbiont identity varied among species more than sampling site. Seriatopora hystrix was consistently dominated by Durusdinium, a symbiont generally associated with stress tolerance, potentially reflecting differential survival following recent marine heatwaves. Porites heronensis hosted a potentially distinct Cladocopium lineage. Acropora lovelli and I. cuneata predominantly hosted Cladocopium goreaui or Cladocopium "C4". Our findings highlight the importance of characterizing coral-algal associations in multiple locations to better understand spatial variation in host-specificity.