New insights into the microbiome composition of Mediterranean and Maldivian coral species

In the last 15 years, coral associated microbiome emerged as an extremely important research field for its key role in the health-status of corals and their potential response to diseases and environmental changes. Despite these evidences, nowadays our knowledge of microbial communities associated with corals remains remarkably scarce, especially in the Mediterranean and Maldivian area, due to the novelty of this topic and the difficulties related to the analysis protocols. In this context, this study aimed to describe and compare for the first time the microbiomes associated with three species of scleractinian corals: two from the Mediterranean Sea (Cladocora caespitosa and Madracis pharensis) and one from the Maldives (Herpolitha limax). Taxonomic analyses were conducted by Illumina sequencing of the V5-V6 16S rRNA hypervariable regions. From the analysis of beta diversity, calculated on the relative abundance of the 19 most abundant families, it was possible to infer the existence of two distinct microbiomes, one on the Mediterranean coral specimens (mainly unclassified Rhodospirillales and unclassified Poribacteria) and the other one on Herpolitha limax individuals (mainly Burkholderiaceae and Nocardiaceae). Moreover, the microbiome of healthy and diseased (Brown Band Disease) individuals of Acropora tenuis collected in the Maldives was analysed, revealing the presence of families as Nocardiaceae and especially Moraxellaceae (Acinetobacter sp.), already cited in another work as the main trigger of BBD in Acropora sp. Further studies are needed to better understand how the different biotic and abiotic factors shape microbiome composition; this information could be crucial to define the microbiome role in coral survival, especially in the modern context of global climate change. Furthermore, these findings could also be used to improve the main mitigation tools currently used to cope with the problem, such as coral restoration.