Photoendosymbiosis of the Blue Subtropical Montipora Corals of Norfolk Island, South Pacific

Corals exhibit complex and diverse relationships with dinoflagellates of the family Symbiodiniaceae. Montiporid corals within Norfolk Island's shallow water lagoonal reef systems have been observed to turn a deep fluorescent blue during winter, suggesting potential environmentally driven changes to their photoendosymbiosis. Here, we investigate the photoendosymbiosis of blue <i>Montipora</i> sp. corals over a year-long study, demonstrating that photosynthetic yield and Symbiodiniaceae densities vary seasonally, with the lowest photosynthetic yield occurring within winter periods. We also provide the first characterisation of Symbiodiniaceae species associated with corals from Norfolk Island, identifying blue <i>Montipora</i> sp. as predominantly associating with <i>Cladocopium</i> (formerly Clade C) genotypes (<i>C3aap</i>, <i>C3ig</i>, and <i>C3aao</i>). Finally, we also report on the impact of recent bleaching conditions (March 2024) on blue <i>Montipora</i> sp. photoendosymbiosis and find the genera is susceptible to increasing sea surface temperatures. Our findings provide insight into the unique biology of subtropical corals within this remote reef and the susceptibility of corals in the region to increasing sea surface temperatures.