Metagenomic Exploration of the Effects of Depth and Temperature on the Microbiome Structure and Function of the Gorgonian Eunicella Singularis and the Cold Water Scleractinia Desmophyllum pertusum = Exploració metagenòmica dels efectes de la profunditat i la temperatura sobre l'estructura i la funció del microbioma de la gorgònia Eunicella singularis i el corall d'aigua freda Desmophyllum pertusum

Abstract

[eng] Corals are crucial to marine ecosystems, offering varied habitats for microbial communities that, in return, provide essential compounds for coral health and resilience. Numerous factors, such as coral age, species, health, and environmental conditions like depth and temperature, influence the structure and function of the coral microbiome. Disruptions in the coral-microbe relationship can lead to infections, bleaching, and increased coral mortality. Understanding these influences on the microbiome is vital for deciphering coral-microbe interactions. This thesis explores the coral microbiome concerning depth and temperature variations using highthroughput meta-omics approaches. Our analysis of the influence of depth on the Mediterranean coral Eunicella singularis revealed significant taxonomic and functional differences between shallow and mesophotic zones. Shallow water colonies prominently featured Symbiodinium, essential for photosynthesis, while its absence in mesophotic zones indicated a shift in microbial community composition. In mesophotic water colonies, the microbiome exhibited higher abundances of genes related to carbohydrate, energy, amino acid, cofactor, and vitamin metabolism, suggesting adaptation to diverse nutrient sources and enhanced nutrient availability. Metagenome-assembled genomes (MAGs) analysis identified key taxa, such as DT-91 (Order Pseudomonadales) and Endozoicomonas, involved in nutrient recycling and vitamin production, underscoring their role in maintaining microbiome fitness and coral resilience. Further research is needed to determine the active expression of these genes and their impact on coral health. Additionally, our study on the cold-water coral D. pertusum under prolonged thermal stress revealed significant changes in the microbiome's taxonomic and functional structure. Elevated temperatures correlated with an increase in microbial taxa such as Rhodobacterales and genes associated with carbon metabolism and secretion systems, potentially destabilizing the microbiome and promoting the growth of opportunistic pathogens. Furthermore, we observed a rise in genes related to diazotrophic activity, including denitrification and nitrification, which could disrupt nitrogen cycle balance and heighten disease susceptibility. These findings highlight the importance of environmental factors in shaping coral microbiomes and emphasize the need for further research on gene expression profiles to better understand microbial activity and regulation under stress conditions. Such understanding is crucial for developing strategies to enhance coral resilience in changing environments.