Abstract Massive corals, like some Porites species, are key reef-builders and provide critical habitat on coral reefs. Porites often exhibit high tolerance to stressors, like warming from climate change, although the drivers of these patterns are unclear. Using high-throughput sequencing, we examined whether associations with Symbiodiniaceae underpin this tolerance. Symbiodiniaceae diversity and community composition within two massive species ( Porites lutea and P . lobata ), whose identities were verified through morphometric examinations, were characterised across 200 km of Ningaloo Coast World Heritage reef, Australia. Contrary to previous studies showing low diversity of high-fidelity algal symbionts, we found variability within Cladocopium , which differed significantly across host species and temperature metrics. P . lobata communities exhibited greater dispersion than those of P . lutea , suggesting higher among-colony variability in symbiont assemblages. Differential abundance analyses revealed both taxon-level and within-taxon (ASV-level) host specificity, including contrasting enrichments of closely related C15 variants between hosts. Locations with broader temperature ranges and a greater number of accumulated heat stress events (measured as Degree Heating Weeks ≥ 8 °C-weeks) exhibited significantly higher prevalence and abundance of thermotolerant Cladocopium C116. Moreover, the effects of accumulated heat stress (DHW ≥ 8 °C-weeks) on symbiont community composition appeared to be host dependent. These lines of evidence suggest that Porites may hold greater capacity to vary, and by extension modify, their algal symbiont community diversity than previously thought. This flexibility could contribute to the genus Porites’ long-term evolutionary success and relative resilience to global marine heatwaves.
Kraemer et al. (Mon,) studied this question.