First insights into bacterial and microalgal endosymbiont communities of various coral morphotypes from Maldives

The Maldivian Archipelago is home to valuable coral reefs that have been extensively studied for their ecological diversity. However, the diversity of the microbiome in Maldivian corals remains largely unexplored. In this study, the microbiota compositions (including both algal endosymbionts and bacteria) were investigated for the first time across various coral morphotypes sampled in May 2022 from four Maldivian atolls (Ari, North Malé, South Malé, and Rasdhoo). Coral and gorgonian specimens were collected via scuba diving at reef sites located on both ocean-exposed reefs and lagoon sites, across various depths (0-40 m). Surface seawater samples were also collected near coral assemblages. Metabarcoding analyses were performed, targeting the 16S rRNA gene to assess bacterial composition, and the Internal Transcribed Spacer 2 (ITS2) rRNA region to evaluate microalgal endosymbiont diversity. Generally, the bacterial communities associated with corals exhibited significant diversity, which was primarily influenced by coral morphotype rather than depth or geographic location. These communities were also markedly different when compared to those found in seawater. The three most abundant bacterial taxa in coral samples were Proteobacteria (ranging from 10 to 95%), Bacillota (formerly known as Firmicutes, ranging from 5 to 10%), and Planctomycetota (ranging from < 1-30%). Most Symbiodiniaceae belonged to the genera Cladocopium-C and Durusdinium-D (> 90%), while host specificity was observed for variant types. Overall, this study provides first insights into the structure of Maldivian coral microbiota, which could be crucial for monitoring the health of local coral populations and predicting the potential impacts of changing environmental conditions in the region.