Coral reefs worldwide confront escalating threats, characterized by increased sea water temperatures due to climate change, resulting in mass coral bleaching events. Endosymbiosis with Symbiodiniaceae algae and bacteria is fundamental for coral survivorship and adaptation to environmental stressors. However, how the spatial-temporal dynamics, inter- and intra-specific difference influence these communities is still not clear.

In this study, we present a multi-site, temporally resolved research focused on algal symbiont and microbiome characterisation of two common reef building coral species, Porites lutea and Acropora muricata, in three different Maldivian islands (Magoodhoo in Faafu Atoll and Thudufushi, and Athuruga in Ari Atoll), across the 2024 mass-bleaching event. Specifically, 80 colonies were tagged and sampled before, during and after the bleaching event, and the different fates of bleaching, recovery, or mortality were identified.  In addition, using ITS2 and 16S metabarcoding, we characterized the diversity and composition of both symbiotic microalgae and bacterial communities, aiming to identify community patterns associated with coral survivorship and post-bleaching recovery. We analysed data using custom bioinformatic pipelines, specifically using SymPortal for the taxonomic assignment of Symbiodiniaceae. We evaluated temporal and special patterns in microbial and algal symbiont density, focusing also on functional profiles.

Our results showed as the two different species displayed a different bleaching susceptibility and even within the same species, corals from different locations responded differently to the temperature increase. Moreover, community-level analyses revealed contrasting patterns between bacterial and Symbiodiniaceae assemblages, with spatial differences among locations emerging more strongly than temporal variation across bleaching stages.

Overall, our findings highlighted as both local condition and coral’s endosymbiosis may play a role in coral holobiont tolerance and resilience. This study sheds light on the role of these symbioses in tropical corals and how their reorganisation after extreme thermal stress may impact their health.